I know that i can run the send in the fx loop from my marshall vs100 head to the input of another power amp. can i run it into the return of the fx loop of my randall 300w head?
Yes. You can run the preamp of the VS100 into the FX return on the Randall.
Framus Cobra
Marshall JCM 900
peavey 5150

Mesa 4x12
and that will use the preamp of the marshall and the power amp of the randall?