First post here since many years, please be indulgent (not a native english speaker).
If I had to create a new format, especially a "vintage redux", I would take an entirely different approach. Let's take this step by step :
What's the problem ?
-> There are fewer and fewer vintage players, especially paper players. Some fresh blood is needed.
What can we do to solve it ?
-> Bring in new players. MTGO has already failed doing this. Old School only brings back a few old players with old cards : no fresh blood at all.
Where can the potential new players come from ?
-> I guess the most plausible is legacy players. Explanations follow.
Why would they want to play vintage ?
-> To play with broken card that are banned in legacy. Each time I discuss with a legacy player, they take an interest about tinker, oath , yaugwill, necro...
What keeps them from coming to vintage ?
-> The answer is always the same : "I hold no chance without P9, and it costs way too much. I have no interest in playing dredge or fish, I'd prefer to play with broken cards, necropotence, etc. And well, I just don't want to play with proxies, that's no fun at all".
How can we make them step in vintage ?
We create a new intermediate format, that takes into account the price/availability of cards.
Fondamentally, that wouldn't be hard to create. We already have a banned list (ante, etc), and a restricted list. The only necessary step would be to move some cards from the restricted list to the banned list. These cards would only be the "legendary and unobtainable P9", + whatever is warping this new format. I would especially monitor bazaar and shop.
Bazaar : dredge might be just too powerful for this format.
Shop : this may be too powerful as well, but I guess shop decks can run without it.
Please do not brush this suggestion off too quickly. Please take the time and give it some thoughts. The players I target are from legacy : many already own FoW, bilands, fetchs, etc. Our non-P9 broken cards do not cost so much (except bazaar, shop, LoA, etc - many are under 100$, most cost next to nothing). This new format, if it became popular, would not impact the P9 prices. On the contrary, if players are attracted to vintage-like formats, more people will build a deck and want to add in power, that would sustain the prices, so no worry for P9 owners.
Maybe a good threshold for the ban hammer would be cards over 100
I can't even start to imagine the unique metagame that can come from this. This would be quite different from vintage or legacy as we know them. In short : yes, that would make "vintage unpow" a whole format.