As your profile doesn't say where you live I don't know which voucher you are talking about. I assume the Chase, which I don't have any experience in, so I'll tell you about the AmEx, which I do.
Yes it is possible to book different classes on the outbound and the return, and it is possible to book different routes so long as they are in the same zone (for example YVR and SEA/JFK and BOS).
I guess you could use it one-way, but why would you?