Would you buy a tube amp if you were unable to use vintage tubes in it ?

Yes, 100% - and this is the situation for basically any power amp that uses a quad or more of KT88+ tubes. Or anything with 6H30 - which is an excellent tube; arguably better than alternate types with vintage options (6922). Vintage tubes are great, but new tubes are better than no tubes.

Plenty of very nice modern 300B, 845, EL34 tubes. I'd definitely have an issue devoid NOS for most others.

It would be interesting for me to compare my VAC Avatar SE full of vintage Mullard and RCA tubes with later VAC integrated full of Chinese/Russian tubes.

"new tubes are better than no tubes". This is something that would be very hard to argue with. I certainly won't. Yes, modern amps should sound good with whatever new tubes are available, but..even Kevin of VAC himself recommended vintage tubes for driver and power output tubes. Surprisingly to me he suggested  I stay with selected Chinese 12AX7 for preamp. I of course did not, though I am sure he knows what he talks about. Besides the sound quality I also want reliability and longevity.

Why would you not be able to use vintage tubes??

Yes I would.