I mostly play metal and occasionally cleans with my JP-2C through a vertical metal grill Mesa 2x12 halfback that has a VS-12 in it, so hopefully I can be of some assistance.
A lot of guys don't seem to care much for the humble VS-12 because its not a powerful nor bright speaker. Mine certainly isn't. It's smooth, subdued and mellow, and it can easily be overpowered when improperly mixed with higher wattage speakers. In your cab's current configuration, you're probably not really hearing the VS-12s over the C90s at all, so your 4x12 halfback is essentially acting more like a raised open-back C90 2x12 cabinet rather than a proper 4x12. IMO, the VS-12 isn't suited for use in a closed-back cab. They simply don't bite hard enough or push enough air, and for that reason, I don't think you're getting the most out of that speaker or your cab. Unfortunately, this is why VS-12s often get written off as a mediocre speaker that should be automatically replaced.
However, I believe the VS-12 has its place, specifically in the open-back upper position of a Mesa halfback cab like yours. They belong there. That is their niche. Here's why. The VS-12 is a quiet speaker, so putting it in an open back position where it sounds louder and more present makes sense. Having the quieter speaker in the louder position and the louder speaker in the quieter position will have a balancing effect and benefit the overall tone. And because there's no harsh upper-mid spike, the VS-12 is particularly suited for angling up towards your face without the icepick effect. It just adds some fullness to the crunchier, beefier speakers doing the heavy lifting in the closed position down below while keeping things nice and pleasant. In this application, they work extremely well.
But you will likely need a pair of powerful speakers with some bite in the closed bottom to complement your VS-12s if you intend on playing metal. Obviously, you'll want to try your C90s in the bottom first. Keep in mind that in doing this, your cab will be a bit darker now, so you may need to adjust your amp's EQ back to taste. If you like the C90's crunch tone in the closed position, then you're done. But you're still not feeling it, if the cab could use more aggression, consider swapping your C90s for something like V30s, instead. Personally, I prefer maximum bite out of my closed cab speakers, so V30s are my go-to.
For my 2x12 halfback, I currently run it with the VS-12 in the open top and a V30 in the closed bottom. With a JP-2C, this combination works nicely with both high gain tones and pristine cleans, and I'm extremely picky when it comes to speakers. When I bought my 2x12 off Craigslist for $75, it had the VS-12 in the bottom and a destroyed Jensen up top. I replaced the Jensen with a Chinese V30 I had lying around and it was WAY too bright, and the VS-12 was completely drowned out. Swapping the speaker positions did the trick. It's a great sounding cab now, and this experience helped me understand why these cabs were built as halfbacks in the first place.
