"Vintage" is abroad term, so shop around to get a sense of what you like and the range of prices. (My fave: Victorian mourning jewelry, in which a widow sealed a lock of her dead husband's hair. It's all about history, emotion, and character.)
Get to flea markets first thing in the morning, right when the door opens. You'll see huddles formed around the best booths. Those dealers will be sold out in two hours and gone by 11:30!
What dealers will kill me for telling you:
They have two prices: one for retail, one for the trade. So don't ask too many questions. If you see a piece you want, say in a business like tone, "What's the dealers' price? I'll pay cash." Cash will get you a nice discount, and it's unlikely you'll be asked to prove you're in the industry.