Another thing to consider when wanting 9 major performance out of a PCC is the OAL of the round. I haven't seen a single USPSA open gun shooting 9 major using a load that would even fit in a standard pistol magazine (certainly not a Glock mag like the vector uses), much less chamber in a standard barrel. To get major power factor, you are filling a 9mm case with so much powder to the point where you can't safely seat a bullet to a standard OAL. Now, that is not to say it would be impossible to do, or that nobody has done it before, but I have not seen it. Perhaps a commercial loading outfit using non canister-grader powder build specifically for a 9mm major load could work something out, but I have already seen enough issues with 9 major, and that is out of guns build to shoot them.
I also have a variety of 9mm PCCs from 4.5-16" barrels. I think it is reasonable to assume that hotter ammo in a blowback gun requires a commensurate increase of bolt/buffer weight. Yes you can eek out a few more ounces by adding tungsten inserts inside the bolt, and using an extended steel buffer with tungsten inserts. I just don't want to be the one experimenting with it, and I certainly don't want to be shooting any large quantity of 9 major through am AR9 blowback gun. The Kriss, still being a blowback gun, is still bound by physics, and just because the weight is being directed down, doesn't change the fact that you need more weight to safely keep the gun in battery long enough, and the Vector isn't a bastion of customization when it comes to bolt and buffer weight.
It seems that you are looking for this project to become some sort of defensive gun. For me, at the end of the day a defensive gun must be reliable. I think this is the sort of idea where the juice just isn't worth the squeeze. All of this is just my opinion, so that that for what it's worth.
Finally, if you are really looking for a compact 'subgun' with greater performance, why not blackout? I have a 7" blackout that is a little lighter than the SBR vector, and about the same length with the stock unfolded. Then you get the supersonic performance that beats 9mm, major or not, by a long shot, is more reliable, and also gives you the subsonic option should you want that.