This is going to sound like heresy (pun intended), but maybe all you really need is a 4x12 cabinet loaded with efficient speakers. You obviously love the sound of your amp - you just want more of it.

I could spend a lot of time recapping past discussions on the subject (Search is your friend) but I will settle for reminding you that you need to put a LOT more power into a given speaker in order to make it sound noticeably louder. Doubling the power to 30 Watts in your case, would be barely perceptible. Low-power speakers tend to be more efficient, which is why I suggest a multi-speaker cab may be the ticket.

See if you can find some specs on the speaker in your amp. The efficiency is usually given as "X dB SPL at 1 meter at 1 kHz at 1 Watt". If that number is something like 90 dB, you're golden. If you can find a speaker set with 10 dB better efficiency, that would be equivalent to getting 150 Watts out of your lil' VOX. Send an e-mail to Ted Weber at his Weber VST website and see what he can come up with for you.

I use a 15" Weber California in the 1x15" combo I made out of a drowned 60 Watt Ampeg VT-40. It kills. Seriously. I can just tolerate being in front of my 120 Watt 2x12" Ampeg VT-22 combo, but I have to turn the 'little' amp down or wear ear plugs.