I haven't had a chance to put my hands on a real Vintage Barbie, they are not my favorites, i have said it before, my passion are on Tina Turner decade...But, I would like to have one, sure!
When time comes, I may not be able to get a "new" one, maybe undressed, with "no good" hair and things like that. Well, you can still make sure you look at some important things before getting one. Would be wonderful if you know the doll you are buying, the year, the model, and more, if possible, which one it's, then take a look on the condition of the hair, if it's the same style, if had been cut or changed; the paint condition (face, nails), body (arms, legs, neck and head - if they're loose), don't forget the ears, some of them have a greenish on it. So, depending on all of that you can see if it's worth to pay the price asked or if you can actually get a "not so good" one and have it restored.
I found some pretty good websites about it, the restorations are fantastic!
Pictures from: http://dollrestoration.com
This is just one that i found, you may be able to found some other people and maybe compare prices and see what it's better for your pocket and collection.