You are right that the Vision Run-Time Engine will be sufficient to run IMAQ for USB. This is a different requirement than other IMAQ drivers however, which do not require Vision or the Vision Run-Time Engine to execute. You should note, however, that the Vision Run-Time Engine also requires a license to activate.
I am getting the same error code on a XP system and VISTA system by the examples (imaqUSB examples),by means of vision library functions (LabVIEW 8.5 FDS, Vision 8.5)
What are the errors that you are getting, specifically? Also, when do they occur? It sounds like what you are seeing might be different from what has already been seen on this thread.