I've read some things in some vegan diet books regarding higher calorie intake that might help you, believe it or not:
1) Add olive oil to vegetables.
2) Get lots of protein - legumes like soy, black beans, chick peas etc. are good.
3) Add jelly to your toast.
4) Lots of complex carbs - whole wheat, brown rice, etc.
These are just little things that you can add to your daily routine without feeling like you're eating when you're not hungry.
You have the option of "drinking your calories." Try to drink half way healthy things like protein shakes for weight trainers and natural fruit juice. I've known a few people who put on a lot of extra weight (and not necessarily muscle) by using those protein shakes...
You must, as I'm sure you know, do aerobic exercise and lift weights. You may be one of those people who are predispositioned to being thin. In which case, try to eat healthy and don't worry about it =)