Did you write the instrumental? I like it a lot.
Do you use EQ on the vocals or anything? Sounds like there is a lot of mids, but little presence area and treble (a trend I have noticed of many a vocaloid mixes).
If you have access to an EQ, try boosting in the 2-4k area with a parametric, and use a hi-shelf to bring out some of the "air" over 10k.
If you don't use EQ, then ignore this advice, but it will really help for clarity (honestly, I find it essential).