I see three possibilities:
a) she doesn’t like the food as much, so she’s eating less. Can you supplement it with snacks you know she likes? Some older people use meal replacements (Ensure); I don’t know if any are vegan.
b) in the smaller portions older people eat, the food doesn’t have enough calories to keep her weight up. Can you ask the facility to enrich it (more margarine for the bread, desserts)? Can you supply cheese substitutes, nut butter (if she can digest it), etc? The doctor is right, it’s important for her to get enough calories.
c) At the age of 85, losing weight could be a sign of her declining, and it has nothing to do with the diet. Try to figure out where she is about this anyway.

Another interesting day at the nursing home--scary, but intersting. Early this morning the dietician mentioned that a no-salt/no-dairy diet would be nearly impossible to implement. I had realized no-dairy might pose difficulties because our culture is so accustomed to cooking with dairy products, but I was shocked to hear that a no-salt diet posed a problem in a nursing home, a facility designed to care for elderly and ailing people, many of whom have heart conditions that call for no-sodium or reduced-sodium diets

This evening I learned something even scarier: The palliative care philosophy of the home extends to dietary considerations! I was fully in agreement with the concept of palliative care--care which promotes comfort but offers no medical interventions designed to prolong life--until I discovered that healthy eating, which prolongs life, is not favored. A nurse explained this to me. Dietary considerations are governed, not by nutrition, but by what the residents like to eat. That explains the bacon! Deserts are made with Splenda so that diebetic residents can enjoy sweets with every meal.

There's one problem with palliative nutrition: It doesn't work. If the forget-about-healthy-foods-and-just-eat-whatever-makes-you-happy approach worked, the nursing home residents where my mom lives should be the happiest people on earth. They're not. Many of the residents complain daily of aches, pains, and illness; others stoically avoid complaining by saying 'I'm doing a little better today.' The residents suffer from lack of mobility, amputations, mental disorders, failing eyesight and hearing, declining communication skills, confusion, and boredom. When these elderly people decline to the point that they can no longer feed themselves, their 'happy meals' are put through a blender and spoon-fed by an attendant.

I also learned that the diet served by the nursing home is not really prepared by the kitchen staff. The meals are bought as a package from a distributor. Meal content is governed by the FDA.

Right now I'm wishing for some palliative care FOR ME because I'm getting real uncomfortable. . . .

Ah, the adventures of vegan travel. Yep, been there. I still get a little gullable at times when they say "Vegan, no problem!" What would we do without Clif?
Not sure if you tried these, but I always take Tasty Bite meals with me. They don't need refrigeration and keep for 18 month (before opening.) They come in a pouch and are ready to eat. I still always have my stash of Luna, Clif bars and nuts but Tasty bite's are good for those times when you want something other than the 100th Clif bar :o).