Well, at the time I wrote that package vector was kind of out in left field and considered a 'big' dependency by many people. How perspectives change. ;)
It was originally written because Data.Vector.Vector used to have no instances for Foldable, Traversable, Monad, etc. and Roman at the time was against adding them, as they had comparatively low performance.
It accreted the rest of the instances necessary for working with my other packages after it had already come into existence, largely due to the fact that my packages at the time were all struggling to have a separate Haskell 98 core, and adding Vector instances immediately destroyed that ambition.
Eventually Roman relented on adding the instances for the base classes, and I was left with a hollow shell of a package full of orphans.
Moreover, as time has worn on the noises of people who want a simpler dependency structure resonates better with me than than the noises of folks who want me to maintain 4x as many packages to maintain distinctions that literally nobody is using. So I've started collapsing many packages together (many of my 4.0 updates were for this purpose).
Finally, vector moved into the platform.
So upon reflection, all of the reasons for the creation of that package are out dated. I'll look into just doing the dependency-inversion and moving the remaining instances for my classes out to the libraries defining them when I can muster that much willpower. ;)
Another package I had orphaned a bunch of instances on was aeson. Now that that too is platformed, I feel less guilty about depending on it directly.
tl;dr Rambling historical musings on why my vector-instances package is now a bad idea.
I'm just a flat +1 on this proposal now, no caveats.