Just for fun,
as far as I know, the SD1 is the only arranger that plays back midifiles using the internal sounds instead of a dedicated GM sound bank; this explains why, if I play the same midifile first on my SD1 and then on my VA7, the difference in sound quality is really stunning, especially for what refers to the drum track.
Maybe, if the SD1 had a dedicated GM sound bank, this vibrato problem would disappear, but frankly I am more than willing to put up with problems like these, given the sound quality I get in return.
Like Roel suggested, you could simply try a different sound until you come up with something you like. If you still feel really disappointed, try to play the midifile on another keyboard and you will love your SD1 again.