Ah, the old question: How do I make the vocals heard over a band with a tiny PA? It's not always easy.
Compression won't help you; it may actually make things worse by making feedback more likely. It sounds like you're using underpowered PAs, and if you want the vocals to be loud enough you'll need the band to play more quietly. But the band has to want to turn down for this to work. Small PA sets can not make a vocal loud without feeding back; it's the nature of the beast.
Getting the band on-board
Many bands will tend to play louder as they go along, people turning up their amps all the time as they go. Make sure everyone is on the same page about keeping the volume down. Appoint a band member to be the "gain police": When (person) says "turn down" or gives you a sign to turn down, you do it. Whoever's doing the mixing would be a good person to do this. If people won't turn down as needed, you may have a problem. Have a talk with them.
On-stage amps take control away from whoever's mixing the band, and are considered A Bad Thing by most live engineers. And, in general, you'll want to get the vocalist loud enough to be heard over the drums. I'd consider doing that first during sound check, only then bringing in the rest of the band. It'll illustrate the problem to everyone.
Sound check
Good sound starts with learning the room. When you get to a room, walk around, listen for trouble spots likely to cause feedback like walls of marble or lots of glass windows. If you set up your stage area in a carpeted area, do so.
An empty room (i.e., no audience) has different acoustics than a full room. You can learn to compensate for that over time.
Before you even set up the stage, strategize. How can you set up the mains to minimize feedback? (See the "Mics" section for more on that.) Will the band be able to hear themselves? Can the amps be turned away from the singer so she can hear herself better?
Mics
Placing a mic carefully can help a lot. Make sure the vocal mic isn't pointed at the mains. Where possible, put the mains in front of the band, facing the audience. Get used to playing without hearing yourself well. Some singers can't do this, and your vocalist may want to invest in an in-ear monitor if she needs to hear herself well to stay on-pitch.
When you set up, intentionally try to find what frequencies make the mic feed back so you can avoid them later.
What kind of mic is the singer using? A more directional mic left on a stand, an SM57, will make it easier to turn the singer up without it feeding back.
Amps
Many tube amps present a challenge: It's tough to get a good tone unless they're turned up VERY LOUDLY. I'd spend some time with the amp, trying to find that sweet spot of low gain and good tone. The same applies to the bass amp. On the guitar amp in particular, try removing any effects or turning them down. You may have to sacrifice some yummy tonal goodness to make this work.
If you can't turn an amp down, you may want to get another, smaller amp. If you have monitors, the bass can even use a DI box feeding into the PA, forgoing the amp entirely.
Drums
If you can, I'd do as little as possible with the drums.
Getting a drum kit to be quiet is difficult. It's easy to say to a drummer, "play quietly", but that can be tough to do. Some drummers are capable of playing very quietly indeed, some aren't. You can try damping the drums, but be careful: It'll make them sound different.
Conclusion
All good sound is a compromise, and you need to make a few decisions and trade-offs. Do a little reading on sound reinforcement, which is amplifying a band to be as loud as the amps and drums. Good luck!
