Salvete Omnes

To clarify, this is not at all canon, nor is this a theory. I’m just having fun.

For some context, I've never viewed Vaati in Minish Cap (MC) as an "evil guy", just a guy who made bad decsions. If you've read the manga, you would remember that in the end, Vaati turns back into a minish and repents for his actions. I know this is just basic happy-ending story telling for kids, but I always chose to interpret it as him claiming that the cap (magic wish-granting cap that Ezlo made and Vaati wore) twisted his want for power.

Expanding on that, Ezlo was not a perfect teacher, nor prepared to raise a kid (yes my Vaati is orphaned). Vaati dealt with bad thoughts, mental issues that I'm not qualified to diagnose, and paired with a creative mind that ended up giving to some inventions and behaviors that Ezlo viewed as "upsetting". He would state this plainly and it would send Vaati into a spiral of denial, trying to prove that he was a not dangerous or unstable individual. I'm not saying that Ezlo is to blame for the events of MC, but I think he held a role in Vaati's decision making. Vaati is dependant on him emotionally in someway and desires proving his worth.

Extra context done with, let's talk about the wind waker(WW):

Ezlo disapproves of the WW because the winds are a sacred thing. For the Minish, the elements are life in its purest form, and seeing how important the wind element is to weather and plants, I think Ezlo would view "controlling" it as dangerous. Vaati, in an attempt to show him it's not, would try to manipulate the waves of a pond (showing that it could be used for crossing water) but ends up forming a whirlpool and water spouts. Which in turn floods the surrounding area, fueling Ezlo's view even more. (As shown in page 2 of my little comic).

I think this would continue. Vaati constantly trying to prove that his WW is good, while Ezlo believes otherwise.

Bare with me here, cuz this is where I go a little crazy: Vaati imbules a part of his life force into the WW. As stated earlier, Vaati isn't an evil guy. He made the WW with the goodest of intentions, wanting to showcase his love for the wind element while also proving to Ezlo his mastery over it. To do so, he uses a portion of his own wind magic, injecting the WW with his magical signature, and therefore a part of his very being. Those good intentions he held carried over with it.

Fast forward to after MC, Ezlo seals the Wind Waker away, stunting its magic power. Vaati was corrupted, whos to say his creation wasn't either?

Here comes the timeline split:

I won't go over the downfall and child timelines in detail, but long story short: in the downfall era, the WW was used and broken. In the child era, it was corrupted and remains sealed away.

In the adult timeline, before the events of Wind Waker take place, Zephos and Cyclos break the seal on the WW and take it. I have always viewed Zephos and Cyclos as existing only in the adult era (due to how I interpret godly entities in media; they exist because people believe they exist.) so this is the only era where this happens. They do feel the life force residing within the WW. It isn't evil, more like a soft song carried along a breeze, and the power from the WW originates from it. Zephos grants it to a few worthy people over the centuries, who create different songs and continued to strengthen it from the power loss it experienced when sealed.

By the time it is in Link's hands, it is almost back to its original power level. The life force? Gaining some sembalance of sentience. Vaati, in the adult era, is dead, with the decay of the Four Sword with him inside. But a part of him, an uncorrupted part of him, lives on in his most prized creation. Only once gaining the Triforce of Courage does Link begin to hear a voice in the WW. Faintly, he hears someone singing the wind as he plays. It isn't until two years later, after waking Oshus from his dream, that he sees a figure conducting with him.

I don't necessarily think Vaati is alive again, nor is his soul possessing the WW. It's more that a part of his soul is attached to it, and what Link sees/hears are after images reacting to the environment. Think like in Phantom Hourglass or Twilight Princess where souls/ghosts are sorta stuck in their final moments, but can be interacted with. Link doesn't meet Vaati, he meets who he used to be.

Or… I could also say Bad Cop, Good Cop~