I've been all over any fixed caliper brake upgrade, and I haven't found any decent candidates.
Of course you would have to make adapter brackets, to mount the calipers. This is not really a big deal if you've already found the rotors.
The rotors are tough, because of the hubs' bolting to them. Custom hats would be an extraordinary expense, so no big rotors.
The final nail in the coffin was caliper clearance. It really doesn't matter what offset you have, the wheels would have to have significantly dished spokes to clear any calipers larger than the stock floating units.
Baer makes nice floating calipers (think Corvette), that don't require as much clearance, but you'd still be looking for some suitable rotors.
I suppose that the VX brakes are fine for the kind of driving it was designed for. With some quality rotors, pads and maybe some SS lines it could get a little better.