Eat lots, and lots!
If you are lacto-ovo vegetarian you can try to eat lots of eggs (especially the white of the egg) and get some whey powder for extra protein.
Also tofu and setien (i'm not sure i spelled that right) are excellent for vegetarians, having high protein and unsaturated fat content.
If you can get a nice soy + linseed bread this is also helpful as it will have protein, carbs and fibre. I like to make french toast with 4x eggs and 4x slices of soy + linseed bread for a quick easy and tasty 1000 calorie "snack" between meals. ;o)
Potato, rice, pasta, and so forth are also excellent for carbs. Ideally though you would want to be putting on mostly muscle and not so much fat, so don't over do it with high carbohydrate meals in the evening, go for more protein and fibre (legumes for example) in the evening.
As you'll be eating a lot more make sure to do some training so that the extra calories go to muscle and not fat.