#1
Guys I've got Vox vt20+ hybrid amp. It's got 20 watts but it's a hybrid amp. My understanding is that a 20 watt tube amp will be as loud as a 60- or 70 watt solid state. How loud is a 20 watt hybrid amp compared to a 20 watt tube amp?
#2
There's not really a conversion like that. Tube watts aren't really louder, they're just 'more useful,' that is to say, tube amps tend to cut through a mix better and sound good all the way up the volume knob, while SS amps often sound crap 3/4 of the way up. Your amp, for all practical purposes, is a solid state amp.

There are two additional reasons you'll probably notice a big volume difference between your amp and a 20 watt tube amp:
-Tube amps are rated differently for wattage, so a 20W SS amp is 20 watts, but a 20 watt tube amp will actually put out 30 watts or so cranked.
-The speaker in the VT20 isn't very efficient, and it's a tiny 8" speaker, so a proper 12" speaker is going to push a ton more air.