One thing I didn't notice before I uploaded it is that each time it loops, the vibrato is less and less. The difference between the first and second time is almost imperceptible (which is probably why I didn't notice it before uploading, since SPC700 only loops twice before fading out), but on the third time through the vibrato is almost gone, and by the fourth time (and beyond) there's no vibrato at all.
Easiest to hear on the "trumpet" solo starting at 0:10, which then repeats at 1:11 and so on.
This is very strange indeed, and I see no reason why it should stop affecting the channel after looping several times. I'm not very sure it will do much, but have you tried redefining the vibrato commands after the intro /? That might force the vibrato to activate every time the song loops.