It's a long time since I last read about the history of Finnish climate, so I'm not sure if I remember it all correctly. But as far as I remember, 1000 years ago the climate in Finland was more or less the same as in my childhood. In the old port town of Turku there is a museum which says that they've found evidence of imported fruits.

But somewhere around 800 AD there was a period when winters were milder. But I don't know if it was mild enough to grow grapes.

Although, it isn't that far - nowadays they've been growing grapes in the most south-western corner of Åland islands. And some grape varietes might also survive on favourable spots in inland Finland, too. I'd guess they need a warm sunny southern wall protected from harsh winds.