To my knowledge, there are no "essential" supplements. You can get everything you need if you have a great diet. But in my very limited experience, the hardest things to get in a vegan diet are *COMPLETE* proteins, zinc and iron. Multivitamins are your best friend, and flaxseed (along with almost all nuts and seeds) are excellent for the health conscious bodybuilder.
As far as building muscle goes, just make sure you get all your amino acids. The essential ones can be difficult to find unless you take a look at some of the most underrated foods on the planet. Like sunflower seeds, for instance. It has good fats, some carbs and chalked full of protein. It contains all the essential emino acidss (the ones your body cannot make) as well as a large list of vitamins and minerals. The first time I made my own meal plan/meal replacement bars, I was shocked to find out how nutritious they really are!
I lalso like quinoa. It is pretty expensive unless you plan to buy in bulk, but this is another complete protein source with one cup giving you 24 grams of protein and 80% of your RDA of iron on a 2000 calorie diet. One cup of quinoa will make you very full and you may not even be able to eat it all!
About the protein... I hate having this discussion because everyone has such wild opinions...
you don't need that much protein to gain mass. It is about calories mostly. I was in the gym for a short stint of a month, and while taking around 120 grams of protein a day, I was totally fine and making gains. I weight 160lb at 5 feet 11 inches. So to me, proper rest and excellent dieting will do most of the work. All my opinion based on what I observed in my own body. Also I do plan to start taking BCAA which is a great way to stave off atrophy (muscle loss) as well as helping the body to build/heal ripped muscle tissue.
One more thing about BCAA. The three really important BCAA (branch chain amino acids) are leucine, valine and isoleucine. They are essential, so you must eat them, and they make up 35% of muscle mass! I like the fact that they instantly go where you need, but the rest gets sunt to urine. So take some before bed, before or after high intensity workouts or when fasting to reduce muscle loss and increase gains. They say 2-1-1 ratios mixes of leucine-valine-isoleucine is a good ratio. GLutamine is a good one too, just not as musch of muscle is made of that one.