If you're looking at Vox breakup within reasonable volume levels, an AC15 with a Celestion Blue (or Gold or the new AlNiCo Cream) won't do it unless you don't mind killing off the power amp drive (which is half the drive tone of an AC30/15). Speakers also have a lot to do with volume. The Celestion AlNiCo speakers have a sensitivity of 100db. Greenbacks and Creambacks have 97, which means with the same amp, you'd be noticeably quieter, but still not enough to get what you're looking for. I had trouble getting natural breakup from a NMV 5-watt amp at volumes that didn't piss off the neighbors when I lived in an apartment.
There are other options if you're set on Vox such as an attenuator. Some of them sound like garbage, some of them sound pretty good. The best are the ones that take your amp's signal and reduce it to a line level and then re-amplify it (BadCat's Unleash for solid state reamping, Fryette Power Station for tube). What these do is allow you to play whatever amp you want at the exact volume you want, reduce it to a line level, and then reamplify it anywhere between 0-100 watts solid state (with the Unleash) or 0-50 with tube power (with the Power Station). But quality ain't cheap.
If you're not planning on gigging and don't care about looks, I'd get a computer, a good interface, and then a Fryette Power Station and a 1x12 cab with a Celestion AlNiCo Cream in it. With my MacBook Pro, an Apogee Duet 2, a BadCat Unleash, and an Avatar 1x12 with a Celestion Gold, I got the Vox tone at both TV levels and gig levels.
Are you planning to gig it at all?