Can differential USB traces be routed relative to Power (not Ground) planes?
The short answer is you're going to be fine, route it above VCC and make sure you have some VCC to GND decoupling caps near your chip.  Plus your route is pretty short at 600mil, I've seen some people do terrible things to USB routes that still end up working :)

I think the best way to understand this is to consider where your return current will flow.  Current is going to follow the path of least impedance.  In a microstrip's case, when you consider the high frequency current of your USB edges this current will flow back in the reference plane directly below the trace.  It doesn't matter if that trace is GND or VCC, it's the path of least impedance so that is where current must flow.

Now some interesting things, current always flows in a loop.  When that return current, happily flowing along on your VCC plane gets back to your chip it must find a way to GND to complete the circuit.  It's going to do that by the path of least impedance again which hopefully in your case will be the VCC decoupling capacitors you placed nearby.  

That's for the AC portion once your signal has moved to the DC portion your current loop will go back to following the path of least resistance.

Also a lot of people will refer to USB as differential and then cite the fact that most of the current will return in the pair itself.  But in USB 1.0 it's usually just two single ended drivers referencing GND so the current should travel the way I described above.

Finally even when you have a differential pair there will almost certainly still be common mode return current unless you can somehow guarantee that from the source to the destination, through connectors, and routing, that the D+ and D- lines are the same length and never hit any single ended discontinuities etc.

Hope that helps.  There are some good books out there on signal integrity if you want to learn more about this kind of stuff such as Johnson's handbook of black magic, and Eric Bogatin's books.