Mids of Am90 is warmer and smoother than VC02. VC02 has more texture and meat(details) to the mids. Vocals wise, you can hear the sounds of d, ed, k, s, t made by the singer when singing the lyrics.
Overall presentation, AM90 is comparatively warmer and veiled. There is this clarity in the SQ of VC02 that is refreshing. VC02's overall presentation is clean, tight and detailed. Separation and layering is really good for an IEM of this price. Sort of have a 3D or enveloping kind of sound stage, imho.
When listening to Radiohead's Creep and Nirvana's Smells like teen spirit, the presence of hi-hats and the cymbals are much more noticeable with VC02.
Although their highs are where they excel (good quality & quantity at a less than USD 50 price range and a small driver), its also a double edged sword. The highs are pronounced and detailed however at times lack the refinement/control of a higher end IEM, this somehow increases the fatigue level of listening to VC02 coz at there is always a little tinge of sibilance. I wonder if the highs will be more controlled after 100 hours of burn in.
Please take the above impressions with a pinch of salt coz it's subjective!