I’ve done quite some research over the years. I switched to a vegan diet years back and run ultramarathons. There is one supplement that is good for almost all active individuals, but for vegans and vegetarians especially and that’s creatine.
It’s probably the most researched supplement and it’s the first supplement mentioned by examine.com for the vegetarian/vegan stack. To me and for many others it was a game changer.

I have to say, my diet was already good and well balanced. And I tried creatine hesitantly since you get a bit heavier, and that’s not good for an ultrarunner like myself. But I only gained two pounds or so. And pretty much digested my training far more easily.

Hope it will work for you.