I've owned/still own many rack preamps (JMP-1, ADA MP-1, MP-2, Engl E530, Mesa TriAxis), and none really do a good Vox IMO, I had the Tech 21 PSA 1.1 and was not impressed with Vox tones. FWIW, my main guitars are Rickenbacker, and my main amp tones are Vox; and I still have a couple of Vox tube amps and Vox amp tones are what I go for in all my modelers.
I think the Tech 21 Liverpool comes close to the sound of a Vox AC30.
If you're looking for a good 1U rack unit, I'd suggest the Rocktron Voodu Valve. They don't do any of the modelling stuff, but they are so tweakable that you could probably get a good AC30 sound out of it. I bought mine used in 1995 and Rocktron is still making them today, with only one revision to add an online editor ability.