So I want a gigging amp for my indie rock band. Now I was gonna get a Vox VT50, looks like a good modelling amp. But funds are tight and I wondered if I could get away with the VT30 instead.

Depending on the size of the gig or whether you are going to mic it or not its hard to say. Just remember solid state amps don't usually sound too great cranked up very loud.
If you play venues that mic the cabs and has stage monitors, you can get away with a smaller amp. In fact, in that case, I prefer a smaller amp because you can keep the stage volume down and you can hear the mix in the monitors. If you're playing a small venue (really small) and your drummer is not a basher, you may be able to get by with a 30 watt SS amp though I doubt it. See if you can borrow an amp while you save to get something more powerful for gigging. It doesn't have to be a monster, but you do want to be heard over the drums.

From experience, even a 50 watt solid state amp may have some difficulty handling a performance without sound reinforcement. I have a 60 watt tube amp that blows away 100 watt SS amps I have played and sounds great doing it. I have a 5 watt tube practice amp that absolutely smokes a 20 watt SS practice amp I own and, you guessed it, sounds great doing it. Solid State amps, modeling amps are almost always SS, usually do not sound great cranked high. There is often a sweet spot at around 4-5 (out of 10) and the sound quality and definition go south after that.