My DIL still takes prenatal vitamins and my grandaughter is 11 months old. She'd not trying to gain weight but she likes the prenatal vitamins. I think it would be a good idea to take some kind of vitamins and if the prenatal agree with you.....why not!
I take Walgreens A to Z vitamins and since I don't really eat many veggies (scared of them) I think it helps with the nutrition I'm not getting from veggies. Another benefit is taking vitamins makes me hungry.
Another suggestion and something that helped me was peanut butter. I don't know if you can tolerate it but if you can spread some on toast with some jelly or do like I do....toast a waffle and then spray some butter and then spread on the peanut butter....it gets all gooey and melty and is really good.
Keep us posted on how it goes and I bet you will be so excited when you hit 90 lbs. It's only up from there! WOO HOO!