I have a VTL TL6.5 and like it very much - in regards to system synergy, the hybrid design of the VTL makes it compatible with pretty much any amps out there (due to low impedance of the FET drivers).
I've even driven amps with 50ohm input impedance with the VTL's - I don't think you want to try that with a pure tube design.
Can't comment on the VAC as I haven't tried that preamp but in terms of build quality - while I agree the VACs use nicer materials, the VTL's are as bulletproof as they come and have a lot of self-monitoring and fault isolation circuitry that I haven't see in a lot of other gear.
That said, I think once you are at this level, any of these will work for you and it's a matter of sonic preference and system synergy.