I'm sure it's pretty much the same as it is for non-vegan bodybuilders/athletes in general. This means:
1. You either get noticed from placing well consistently at shows and they come to you, or
2. You put together a personal "kit" to showcase yourself and your accomplishments in the sport, featuring photos, training history, contest results, etc. to basically sell yourself as someone they'd want to have as a representation for your company
That being said, I've heard quite a bit about what one can expect from being a "sponsored athlete" - many supplement companies will simply send you a reasonable amount of product, and in return, you'll be expected to be a brand ambassador for them by representing their company as much as possible, be it at shows, on message boards, by wearing their stuff all the time, etc. If you're REALLY accomplished, then you've got a crack at getting an actual check, but that seems to be reserved for those who are getting regular magazine coverage and/or have a big personal following online.
It seems that what you'll be able to get for it will depend primarily on who the sponsoring company is and how far you've gone in the sport. As there are a LOT of bodybuilders out there, and a lot who look great and are hoping to score something free if possible, I'm assuming that companies are pretty damned fussy about who they will work with as far as sponsoring people.
Best bet is, keep entering contests, keep building a personal "brand" around you, your training, diet, performance, etc. that develops a good following and keeps growing consistently, and see where it goes. Keep putting yourself out there entering contests and working to consistently place high against a viable pack of competitors to show to prospective sponsors that you're worth the investment. It's likely easier to get sponsored by a smaller company, particularly one that would be vegan-focused (not sure how tough it is with Vega, but you could try PlantFusion, Sun Warrior, NutriBiotic, MLO, NutraSumma or others who are not huge companies sponsoring tons of athletes), which might make it easier to be noticed and given a shot to prove yourself. Just remember, there's a good chance that most companies are approached with a request for sponsorship every day by someone new, so if you want to get a crack at being offered something, you have to stand out from the pack and make them WANT to have you associated with their company.
And, most of all, make sure you're actually going for something that you WANT to use regularly. Companies can often smell it from a mile away when someone is just throwing their line out to everyone in hopes that they get a bite for something free, and usually, those people go to the bottom of the pile for consideration. Show the company/companies you're going after that you USE their products and that you BELIEVE their products have actually helped you succeed, and they'll notice you above those who are just saying "I want free stuff/free money, gimme something because I think I'm awesome!" Always put yourself in the company's shoes - what are THEY getting out of it by giving you something for free, and, are you the sort of person who has the look/personality they want to use as advertisment for their products? Remeber, nobody rieds for free, and in return, if you can't help that company's sales to increase for whatever you can do to promote them, they won't see any reason to give anything out without expecting a reasonable return on their investment.
15 years involved in skateboarding (where everyone dreams of being sponsored and getting free stuff because it gets mighty expensive) prior to being involved in vegan products taught me a lot about how the sponsorship stuff goes in both industries, best of luck to anyone pursuing it because it doesn't seem to be easy unless you're at the top of your game!
"A 'hardgainer' is merely someone who hasn't bothered to try enough different training methods to learn what is actually right for their own damned body." - anonymous