Best way to break the news gently that I will be unavailable for "chores" after Mom gets vaccinated?

What do you mean by "upsetting the apple cart"?

Do you mean that you can't bear the idea that manipulative mom will be upset, play the "poor me" guilt card, tell all the relatives and neighbors what a terrible child you are?

Do you want to live your life always in her thrall because making her upset or angry would be painful?

So, she gets angry. So what?

The world will not end.

Well, I certainly would not bring it up beforehand. That would be self-sabotage of a major sort.
As Beatty here says often, "There will be no solutions if you are all the solutions." Wait until 2 weeks after your Mom's second vaccine and tell her THEN that you are so relieved; that you didn't want to whine when really there was so little choice in keeping her well, but you are relieved now to be able to return as life as it was". Then do so. There will, of course, be complaints. You already understand that. When she whines it is simply a "so sorry it is difficult to get back to real life, Mom, but it is so lucky that we can".
If applecarts are upset then stop helping her to pick up the mess. Walk away.

"OK, mom, now that you're vaccinated, let's get things back to normal and get your paid help back here to do the stuff you need someone else to do. I have to get back to my job/family/volunteering/life, etc, as well. Now that things are opening back up, we can get back into your old routine."

Don't give her a choice. Either she accepts outside help, or not; but you're availability is no longer going to be something she can rely on.

I suspect you're not going to be the only person who will be dealing with this scenario in the upcoming months. I imagine there are going to be quite a few of these conversations being held in the not-too-distant future.
And Barb is right - so what if she gets mad?

Tell her that you will not be doing all of these chores anymore and that you will make sure the next "helper" that comes has received a Covid-19 vaccine. Or you will make sure she is vaccinated before bringing in new help. Then find her another homemaker/companion and let that be the end of it. Warn her that if she alienates the new help and refuses to allow them in to do their jobs she will be out of luck. Then stick to it no matter how much she complains or laments about it. If she decides to throw the new help out anyway, when her house becomes filthy, there's no groceries, and the pet gets sick she will stop alienating the help. Some people have to learn the hard way. I hope your mom doesn't.

Ooh I’m in a very similar situation where my mum would be quite happy for me to do intimate care when it’s needed. To the extent that said she would sit in her own mess if I didn’t. I would like to get one hour a week care in just in case we need to step it up. I realised I was being manipulated by you and it was very hard but I have finally done it. Sat her down and said ‘No I will not be there to do it.’ You have to come to terms with the fact that you cannot please her and protect yourself as they are opposites. I have also put it in writing to my brother who also expects me to do it. The alternative is that you will sacrifice yourself. Don’t do it be firm and don’t expect her to love you more if you do more. You will be disappointed. Good luck x

I would check with the previous care-givers, to be sure they are still available AND have had or will soon get their vaccinations. If they are not available, you'll need time to vet new people and hope they are as good or better.

Meanwhile, decide what it is you might still consider doing for her, even if only occasionally. It might be best to wait for the time it will happen, which won't give her time to plot excuses, get riled up or whatever. THIS is what it will be now mom, everyone is getting vaccinated and then getting back to the way things were, which includes me. I have a job to do, and will be required to be there, so I CAN'T be doing what your helpers did before. They will resume the old schedule starting on X day. IF she's going to continue ordering food and/or supplies and can't move the bags/boxes, then you need to tell her that she will have to order them so that they arrive on the days that the helpers would be there. Home deliveries can be great, but only if you can manage to bring it all in yourself or have someone dedicated to this task, NOT you! Some grocery items can't be left out, sitting for hours before being put away. Items can also be stolen.

Once you give the cut-off date, you'll have to stay strong and not give in if she does something dumb, like order when no one is there to bring it in or fires the help! If she fires them, have a list of agencies that hire people out, along with their phone numbers and leave that for her. You fired those wonderful people, you find a replacement through these agencies. I *CAN'T* do this anymore, I have obligations that must be met OR I've been run ragged and am ill, whatever you want to tell her, but don't buckle!

What are you willing to do?
Have a general chat about how good it is to be able to get back to some levels of normality - and how much YOU are looking forward to being able to go back to your "normal" life. Tell her you will still be calling in as you did before but of course you won't be able to do things that covid has allowed you the time to do, and ask if she would like assistance in finding people to come and help her.
I guess what I am saying is talk to her with the assumption thing will go back to how they were and this is the normal situation and its good that it will be resumed, don't even entertain the idea that this is not the case, and if she pushes back then just say - it was only every a temporary situation and life has to go back to normal.
The fact that you worry about upsetting the applecart sugggests she is managing to manipulate you NOW, its up to you how much you are going to do and how strong you are willing to be. Manipulators like upsetting applecarts and pleading "poor me" - she'll get over it, just don't guilt yourself on whateveryou decide.

I know it is easier said than done, but you need to be straightforward and firm with your mom.  Politely tell her you have no desire to be her housekeeper.  She needs to do it herself and if that is not possible, she needs to hire it out. 

When you are dealing with a self absorbed person who expects you to do those things without recognizing that you have your own home to take care of and may not have any desire to work yourself to death taking care of their home too...she is not going to take the news well.  She will more than likely throw a hissy fit and you're just going to have to let her do it.

Just tell her you want to be her daughter, not her housekeeper.

Sometimes you have to upset the applecart to get rid of the rotten apples.

Your mother is using you. Inform her that you will continue after a contract is made and she will pay you the going rate ( not minimum wage, professional rate). See how fast she changes her mind.

Be firm but respectful. If she refuses to listen and tries manipulation inform her there will be no contact until she mature and apologize for her conduct and agrees to either pay you or shut up.

Um.

"She says she won't let anyone in the house that could give her covid."

And what makes her think that you can't give her Covid?

Get an appointments diary. For the next couple of weeks, write down the tasks involved in supporting her so that you have a comprehensive list. Source tradespeople who can undertake them. Give her the numbers. Book your next time away and tell her when you're going (this particular trip could be hypothetical/conceptual, by the way. You don't actually have to show her the tickets and reservations, after all).

I have to say, I think you've allowed yourself to be suckered into your current position, and it is bound to be a little uncomfortable detaching from it no matter how delicate your approach. But this too will pass. Focus on the practicalities and just get it done.