Yeah, a standard problem with sampled instruments is that they only work well for a small frequency range. You'd need several samples taken at each octave or even more frequently to get a convincing high-range instrument. Currently, that would of course have to be accomplished using several different vsmp instruments as each one can only have one sample. Bass usually only needs a single octave anyways though, so that's fine.
You should tune this instrument up a half-note to C#3 by the way, as it's currently slightly out of tune with other instruments (you can use xnes for reference). Actually tuning it "up" will make it play at a lower pitch, due to how the mapping is done (you're specifying what note the original recording was made in).
To get a cleaner loop, you can try using a longer sample. Upwards of half a second should be fine for a moderately complex sound that doesn't have a lot of low-frequency action going on. A guitar sound might cause problems as it tends to "filter out" as time goes by from the initial string pluck. It generally produces finite "short" tones as opposed to a violin or flute which can keep going with constant sound character as long as it is "driven" (bowed or blown). Distorted guitars are a bit better off in this department though.