On my first playtrough I sided with NCR and I bought most ammunition from the vendor in the Hoover Dam offices. My question is, I want to side with Cesar's Legion next, is there a comparable vendor or is there only the Gun Runners I can use?

If you keep your great khans faction you can get by far the most ammo from their armory vendor. She also pays retail price for any items you sell. The problem is she usually doesn't have a ton of caps to buy stuff, around 2-3k, but if you buy your ammo and sell your extra gear you can break even if not clean her out of caps. She even has a workbench and reloading bench in her room for your convenience. Her restock time seems to be the same as the other vendors, 73ish hrs.

Gun Runners, although they aren't sided with anyone. I never bought supplies from Hoover Dam though, the Gun Runners usually supplied what I needed.

Get some faction armor if you can spare the weight, then you can still use NCR vendors (I doubt you would be able to make it into Hoover Dam though).

I wonder if a white glove mask would let you fool NCR? It's pretty lightweight.