Is it viable, as a vocalist, to write lyrics and a vocal melody to a song without proper instrumentation (as opposed to a guitarist writing instrumentation, then adding vocal melodies) and then add riffs and other structures later? Have any of you guys done this with good results?

I ask because the only two instruments I know are drums and singing (of course), so writing a chord structure is pretty difficult.
Yes it's easy if the guitarist or whomever you are jamming with knows their scales and theory.

The vocal melody will use a scale, the musician finds this scale, determines the key, and usually the vocal melody emphasises tones from chords employed in that key which can then be picked out. There can be some trial and error involved, as one note in a key can suggest a number of chords but with practice it's relatively easy to figure out a number of chord progressions which will work with any given melody.
And no, Guitar Hero will not help. Even on expert. Really.