@inna Sorry to be so late to this. If I may add a humorous aside?
Keep anything long enough and by default it will become "vintage". 
IMHO, much like the halo around "Cuban cigars", I think that "NOS" tubes are somewhat overrated. "Old" doesn't necessarily translate to "better". Yes they were "made differently" back when, but modern materials and manufacturing methods are unnecessarily demonized. Let your ears be the judge.
Happy listening.