Mom is 79 with vascular dementia. Physically very fit but raising her feet when walking on flat surfaces. Any ideas?

The eyes may be fine but the brain is broken, vision difficulties often go hand in hand with brain disease.

https://www.youtube.com/watch?time_continue=30&v=ZqbxFD2-lsQ

There could be some spatial relation issues that are starting to crop up. I,know in some Lewy Body experiences, the delusions and hallucinations cause the person to think they are "stepping over" things they see.

My first though was an eye related problem and the bi or tri focal glasses.
Second thought..does she do this with both feet or just 1? My Husband had developed Drop Foot and he walked the same way but with just the effected foot. Drop foot as my Husband explained it to me his foot "did not work" he would not flex his foot at the ankle he would sort of pick up the entire leg and go forward. there are AFO's (Ankle Foot Orthotic) braces that might help.
By the way this is almost a gait that you might expect after someone had a stroke. With the Vascular dementia it is possible that she had an undiagnosed stroke.

Now I am going to caution you....
Do not hang on to her.
If she goes down you will as well.
Get a Gait Belt and use that to guide her securely. If she starts to go down do not prevent the fall but guide her safely to the ground. If you can not get her up by yourself call the fire department (non emergency) and ask for a "Lift Assist" tell them no transport to the hospital is needed. They will some help her get up properly so she is not injured and you do not get injured trying to get her up. Most departments will not charge for this as no transport is done.
Is your mom using a walker? If not she might be more secure using one.

Terharv, I wonder if this could be an eye-sight issue even though you had your Mom's eyes checked. Does your Mom wear bi-focal glasses? When someone is wearing bi-focal they are looking through the "reading" section when looking down to walk, thus the vision is blurry.

My own Mom didn't have dementia, but I noticed in areas that she wasn't familiar with walking, she would act like she is sticking her toe into a swimming pool to see if the water was cold... thus trying to feel her way when walking. She didn't do it at home as she had in her brain the floorplan of the house.

My Mom's eye doctor had her try regular glasses, no bifocal, but Mom was stubborn and wouldn't wear them after the first time :P So I don't know if that would have worked or not.... [sigh]