From what you describe i can't see what could be wrong so i plugged in an SM58 to see what happened.
1. SM58 --XLR cable to VF08 BAL input.
2. Trim at between 2-3 o'clock.
3. Track fader at 0.
4. Master fader at 0.
5. Headphone volume at 2 o'clock.
Level just touches the upper line on the display. Any further on the trim and the level goes overboard. I need to be extremely close to the mic to get any levels worth speaking of.
When i track guitar with the VF i use a rack unit to line the guitar. Works, no problem. I also record with a SM57 from time to time, but not on the VF, and the sound is usually pretty loud out of the amp, plus the main mic is placed close to the speaker cone.
I did read a post about someone having problems with the BAL-inputs on his VF08, and he had to have them changed. I don't know what his problem was though.
If you still can't get your levels up, and if you're still thinking about getting a preamp, a good idea could be to bring your VF down to the store and test the preamps with it.
Good luck,
micmac