Firstly, hearing real instruments as opposed to midi playback had me feeling better about ideas I've been doubting for a couple of days.
My concept for this piece was to have a slow, disjunct A section - have the instruments jump around in register, filling in each other's spaces rhythmically, and just being unpredictable in general. I then wanted to have this intensify, only for the piano and vibes to suddenly come together for a brief, trance-like climax, and then to fall apart again and end the piece in the same vein that it started. I had decent ideas for the different sections, but transitional material is always the biggest challenge for me... having a couple of ideas and knowing that they can somehow work together, but not knowing how to get to and from each.
I was given the suggestion to have the instruments gradually come together as the B section approached, rather than to have the "falling apart" nature intensify - this would make for a smoother and easier transition. The idea of having something abstract and arhythmical in one instrument (perhaps solo), and then the second instrument entering with a counter-rhythm that clarifies the pulse and the feel was Dr. Ross' more specific suggestion, and has already helped me come up with some possible solutions.