Rumor has it that the Peavey VTM is a JCM 800 clone...
I assume when you say JCM 800 you mean. 2203/2204. IMO there are a lot of good answers here. I think Lester is on the right track. Only a Marshall is going to have the Marshall snarle and EQ. I have a bogner shiva that captures what you speak of but has no headroom = too much gain. I would caution against too much gain as this will leave behind the Marshall definition.
I would if using a 2203/2204 apply a .68uf cap to the V2a cathode resistor and use a BBE Green Screamer in front of the amp. The cap will add a little smooth gain, the screamer will add more gain than a SD-1 and not add more Mids which is more of a modern metal tone. And it WILL still sound like a Marshall. Good luck.
Just my $.02
David
+1 on Ceriatone. I have the 100 Chupacabra and it does a great traditional 800 tone as well as a badass, pissed off Jose 800 tone (as in way more gain/saturation). It's so close that I was debating which of the 1 watters to pick up, the JCM-1 or the JMP-1 but honestly, the JCM-1 would be redundant for me because the Chupa already nails that vibe so well, and so I've decided on the JMP-1 (if I can find one, lol).
The Quick Rod was another great recomendation...especially first gear.