Does anyone know if a Digitech effects pedal intended for a guitar could be used as part of a vocal chain following a Tube preamp? Or would it just be garbage?
It would depend on the effect and vocal range of the singer. It will sound much better through pedals designed for vocals, but it will still work. Distortion won't sound too great, but delays, echoes, and compression will work just fine. I can tell you from experience, as I run guitars and keyboards through bass pedals and the other way around. Distortions and wahs almost never sound right with pedals designed for different frequency ranges.
See here or search "vocals through guitar pedals".