whilst I'd agree taking the moog out is generally the way forward - as it's got it's own case and power - it's up to you if you are happy paying double for those... maybe in the future when you need more rack space you'll change your mind...
the next module I would get if I were you would be a triple or quad cascading vca... for 2 reasons, 1 you'll be able to mix the outpts of the vco and the moog 2 you'll be able to shape notes from the vco, rather than just letting it drone
you'll probably want a filter next for the vco and then a mult (although I'd be more tempted to get a splitter or stackcables to save space)
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities