| Could be a lot of things... there's a good chance it's something simple and relatively inexpensive to fix.
Do you play with the head on top of the speaker? If so, does this distortion go away if you move the head off the speaker onto something that doesn't vibrate?
When was the output stage bias last checked? My SVT-CL has easy LED indicators for this, I'm not sure about the VR. If it's biased too "cold", it can cause some distortion.
How old is the amp? Maybe a tube is going microphonic. With the amp on and out of standby, take a pencil or chopstick and tap gently on the power amp tubes (they are easier to get to than the preamp tubes). If you hear a noise from the speaker, the tube you tapped is sensitive to vibration and should be replaced. That's a bummer if it's an output tube (I had a bad one, spent $200 on a matched set of Tung-Sol reissues, now I'm happy).
If the VR has "preamp out" and "power amp in" jacks (or "effects send" and "effects return" jacks) try connecting a guitar cord between them. The "in" or "return" jack has a switch in it that can get a little corroded over time, the cord bypasses the switch.
I'm sure others here will have more ideas... Jimmy? | 
