I ride C4's a lot, and i've also spent a lot of time on the Vapour X over the last few months...
The c4 is more C... if that makes sense - you get more slack when you pop than the VapX, but you sacrifice a bit of relaunch.
The Vapour X is a bit more versatile - more depower, easier relaunch, but it pulls a bit harder in the air, and sits further back in the window. It's also got more of a bridled feel.
One big advantage to the Vapour X is the Smart bar - you can make it very short without cutting it, just push the pin, slide the ends off and take out a spacer. I find this helps heaps with freestyle as you can slow the kite down heaps.
The C4 has a nicer feel unhooked - it's a bit more stable, doesn't stall, and the bar itself is comfy and grippy. Depower system on the C4 is very good too, clean and easy to use.
It's pretty hard to choose - the C4 and Vapour X are my two favourite kites at the moment. If you want to be more 'pure freestyle' then choose a C4. If you ride waves occasionally or want everything to be a bit easier, go a Vapour X.

Oh - by the way, the C4 is great at the new school SUK moves