OK, a couple issues to help you with herePity - but this does not explain why a (not so good) wine tasted better after 4 weeks in a weak vacuum. Possibly the vacuum was not so weak after all. If it did not work at all the wine would have transformed into vinegar ...
- Why did it taste better with air? Part of wine development (note, not the same as "aging") is an aerobic process. It all depends on the type of fermentation conditions the wine went through and how the wine was bottled. If the fermentation was "reductive", the wine chemically may need oxygen to stabilize. That can remove some sulfidic characters (think sewer gas), bind with tannin, and lots of other good things. Note, that's different from "aging", which should nearly always be anaerobic. So, exposure to air, even a lot of air, can be beneficial for some wine, or at least not damaging for a while.
- The vacuum is unfortunately weak in the sense it leaves enough oxygen in the bottle to react with the wine. https://www.winespectator.com/articles/do-vacuum-pump-seals-preserve-wine-54662
- Turning wine to vinegar can only happen if a bacteria called acetobacter is introduced to the wine and has the conditions to grow. So just like yeast converts sugar to alcohol, acetobacter converts ethanol to acetic acid in the presence of oxygen. But that takes quite a while - and the presence of oxygen is not enough.
Anyway, I'm glad your wine was nice when you went to drink it!