I should say that 90% of my wardrobe (in the UK) is vintage. There are pieces i simply adore they come from all different places and some of those places are vintage fairs.
1. Set yourself a budget, these days some vendors will take card payments but most only deal in cash. It is very easy to go to a fair and feel the urge to buy EVERYTHING because you will 99% of the time never see it again but you must set yourself a limit or you will find yourself walking away with 100 things and you will wear 2. I always set mine in accordance with how much money i have at the time. I have been to fairs with a budget of 10.00pounds and then i have been to some with a budget of 50.00pounds and i have always stuck to them.
2. It is ok to pay 1 or 2 pounds entry but i would NEVER pay more.
3. Never buy anything you cannot repear yourself, i use this rule because i really cant sew or fix much at all. It may end up costing you a small fortune to make your "bargain" wearable.
4. When looking at vintage make sure there are no stains, because you will probably find that the stain has been around as long as the item and it wont come off.
5. Know your size, i never thought i could wear vintage as i am not super tiny. But i can and find loads of things i like. At some fairs you can try things on, others not so much so make sure you can gauge what will fit and what wont. And if something doesn't fit exactly again if you cant fix it yourself DON'T BUY IT.