The RCA jacks are the CD input, which is the input to the power amp. There is a 1/4" effects loop jack, but inconveniently it's the send and return in a single jack - you're supposed to use a Y-cable to connect effects. Looking at the diagram in the manual, it looks like you can plug a mono 1/4" cable into the effects loop jack and use that as an input - the return is the tip of the Y-cable. But, the effects loop is post-preamp (good!) but pre-cab simulation (bad IMO) so it's not perfect.
Try just plugging the output of your VT into that effects loop. If it doesn't work right, or the cab simulation makes it sound junky, you'll have to use a 1/4"-to-RCA adapter and plug into the CD input. I'm really curious to see how this goes, so do let us know how it goes and what you think of the sound.