Probably the easiest thing to do would be to have some smaller snacks in between meals. A protein shake here, a handful of almonds there, an energy bar... it all adds up. For the big meals, try adding little things that don't necesarily make the meal bigger, but up the calories. In the vegan world, the caloric powerhouse of choice may be nuts and seeds. I like using flax, chia and hempseeds and sneaking them into my meals for a caloric boost + a great source of omega 3s.
Robert lays out some solid meal plans here: http://www.veganbodybuilding.com/?page=article_samplenutritionprograms
It might be helpful to figure out exactly how many calories you are eating now, so you know what to add. I have been using http://www.myfitnesspal.com
for years to track and plan my meals, and would suggest it as a tool to devise your eating plan.
Best of luck!