I know there are some good vegan ones out there, but check with your doctor to make sure there isn't something in them you need to avoid. I'm guessing that you are looking to supplement your protein, to be sure you are getting enough for you and baby.
I'm thinking there is a brand called something like vega (I can't remember the name-its on the tip of my tongue) Its good for vegans. I've been thinking about using it myself, I'm not a big meat eater-former vegetarian- I've always struggled with getting enough protein.
You should ask your doctor, im not sure, your pre natal vitamins give you everything you need