I've not done so myself but would love to go for weekends buying in Europe! I know Holland and France are supposed to be pretty good for vintage in the bigger cities (much as with anywhere cosmopolitan I suppose), and have found quite a few vintage shops reviewed online in Paris and Amsterdam. What I'm not sure of though is the price you'd be paying, as you'd need to know where to seek out the good stuff without paying over the odds..
A quick trip to Paris to go exploring has crossed my mind - would be especially easy from London with the eurostar - and then even if fruitless you can cross it off the list and just enjoy croissants and, well, being in France for the weekend! Sadly until the years of slow accumulation have been dealt with is a big No from my husband, who I think imagines me pulling up to the house with a taxi full of suitcases :D
What I can say is that I failed miserably this last week in Portugal to buy anything vintage (but then I wasn't anywhere remotely big).