All 'small' amps are not created equal. Both of those amps are solid state; due to output rating & speaker sensitivity you may not be getting what you think you are. First of all, most solid state amps are rated at 4 ohms output impedance. If you Vox's speaker is 8 ohms instead of 4, you'll get roughly 15W out instead of 30; halve that again to 7.5W if it's a 16 ohm speaker.
The bigger deal is sensitivity. For example take the Celestion G12L-35 which was standard equipment in a lot of inexpensive amps like the original Marshall Valvestates and MGs. It has a sensitivity of 95db @ 1W/1m, while a Vintage 30 is rated 100db @ 1W/1m. A difference of 5db doesn't sound like a lot, but the difference between a 50W and 100W amp is only 3 db.
You absolutely do not need a 100W head to be heard; in most cases it'll simply be too loud. For bedroom playing even 5W can be too much with an efficient speaker, while 15-20W is generally plenty when playing with a band.