I don't know the percentage, but it's a big group of the younger generation that buys vinyl. This is a picture of record store day in front of one of the many vinyl shops (the Music Mania) of the city where i used to live (Ghent, Belgium). Other shops had similar waiting rows that day:
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This town has about 10 specialised vinyl shops in this city of 250K inhabitants, and many mainstream stores (like the media markt, a very mainstream electronics shop) also sell vinyl of more mainstream music, as it sells like hotcakes.
Many new releases sell out the day of release, even in obscure genres, and often got many represses within months of the release date. And even mainstream arstists press vinyl today. Stromae and Angèle, the two biggest Belgian artists sell their music on vinyl on big quantities, more than cd's (but not more than streaming). It's also better for their income, as vinyl releases earn more money per copy than streaming or digital sales. They are everywhere to buy, on mainstream sales websites like Bol.com (the local copy of Amazon) to local specialised vinyl stores like the Music Mania above.
It's that bad that waiting rows for vinyl pressing plants became ridiculous (6 months is not abnormal), altough their are new plants opening every year and the vinyl pressing capacity of Belgium went up by the factor 20 compared with a decade ago.