Vocoder Question

I think Prosoniq's Orange Vocoder fits. In one mode you can feed in the carrier signal from another VST etc on one stereo channel and the modulator signal as the other channel. There's another mode with a built-in synth for the carrier and a built-in sample player for the modulator too, and I think also a third mode which might be the best for you, but I don't remember using it (I always used the first mode with mono inputs) so I'm not sure: I believe it basically uses the two parts as two separate VST effects so you can route external VSTs in without making them mono and messing around with panning.

However, I may be misunderstanding the question, and anyway I haven't used it at all since version 1.something was the latest and I never used it that much anyway, so please don't take this post as definitive. There's a time-limited demo of v2 at http://www.prosoniq.com/ if you want to play with it and see if it does what you want. (Hm, I see v3 is due in May, too.)

There are probably others out there which do what you want but I've never tried any others.

The Korg Legacy Collection sounds like what you want.

Doepfer's vocorder system allows you to input anything as carrier or modulator.

Most of them do.
The new Waves Vocoder "transformer" does, so does so does the Native Instruments Vocator and i think you can even do it with that Sonige one that comes bundled with the old Cubase 5.1 (not sure about SX) they do:
left channel = modulator || right channel = Carrier

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