I finally got Vienna Imperial working with Sibelius--with one unacceptable flaw. Vienna Imperial does not sustain any long notes for more than 1/10th of a second. Long, legato, sustained notes--each of which should last more than a second--are now cut off with silence after the attack, as if they were short staccato notes.
The problem appears to be located in the Vienna Imperial piano. When I play back the same passages in Sibelius using Basic MIDI sounds, all of the long, legato notes are sustained, just as they are notated in Sibelius.
There must be some setting in Vienna Imperial to restore the sustained legato notes--but I cant fiund it in Vienna Imperial.