To answer the original question, and echoing what's been said already, there is no way to use the vouchers on an internet fare currently.

That being said, I was informed by my local CTO that UA is in the process of introducing electronic vouchers (don't know when it will be done).

Also from the same CTO, UA is actually trying to convert some of the CTOs into internet booking sites (like an internet cafe?) where agents are available to help when needed.