OK, try this if you haven't:
"Restoring the factory settings
Here’s how to restore all settings of the VT-series to their factory-set state.
NOTE: This operation will erase all programs that had been saved to channels,
and will initialize the factory-set programs.
NOTE: This will also initialize the effect and noise reduction settings that you
made in Manual mode.
1. Turn off the power.
2. While holding down the [CH1] and [CH4] switches, turn on the power. When the
BANK and CHANNEL LEDs start blinking, release the two switches you had been
holding down.
HINT: If you decide to cancel initialization at this point, press the [TAP] switch.
3. Press the [BYPASS] switch. The BANK and CHANNEL LEDs will change from blinking
to lit, and initialization will begin. In one or two seconds, initialization will be
completed, and you will switch to Preset mode.
NOTE: Never turn off the power while initialization is in progress."
Also, check and see if it's on noise reduction mode, if it is turn that off. I don't think it's the power level on the back panel but it wouldn't hurt to check, spin it up to halfway. Lastly, as far as I know that model doesn't have a tuner but if it does, switch it off.
There's a troubleshooting guide in the manual, among other things it says to make sure there's nothing jacked into the external speaker output, headphone output, or pedal loops.
Not sure if any of these will help but it can't hurt. Good luck, hope you get it sorted! Vox make good amps so don't worry that it's a bad purchase, it might be that there's something wrong but don't be put off!