General

Another vegetarian gainer made a topic a few months ago asking for advice on weightgain and got a fair few responses, you can check it out here:

fantasyfeeder.com/forum/posts

Another one here too:

fantasyfeeder.com/forum/posts

Depending on what your restriction is there's still plenty of vegetarian fare that's pretty fattening. Peanut butter, a gainer staple, is one.

I would love to help you gain! Being a vegetarian does make it harder to gain, but it's not impossible!
I am assuming you are a vegetarian for moral reasons versus health reasons correct?
Do you eat dairy and eggs? If you are willing to make exceptions for those, it would be much easier, but still doable if you are a strict vegetarian.
Though I am using it for weight loss, I am currently eating a food replacement supplement called Soylent; which is vegan. Doubling or even tripling the standard serving (2,000 calories a day per package of the powdered variety) would get you a long way along the road to weight gain!