A Voyager with a VX351 is great to controll modular synths, but in my opinion the Voyager itself is no substitute for a real modular. If you want a modular, get modular.
The Voyager is the descendent of the Minimoog, which is an all-in-one synthesizer (.... without a fx unit

), designed to be played without minding about technics.
I don't have a vx351 but i use a doepfer dark energy and a diy-attenutor-box to expand my voyager with 2 additional LFOs, an EG and a great VCO. The Voyager and the DE are connected via midi.
The Dark Energy's VCO sounds crystal clear, more brilliant than the Voyagers VCOs, but it's scaling is less stable then the voyager's, especially if you play over 3 or more octaves.
Now I have gained great new possibilities, but most of the time, all i need is built-in.
If someone with a voyager would ask me, what to buy next, i'd suggest a valve bassguitar amp. This is the way to get really nasty!