how will a lrp 4.5 vector do in a rusty?

Good 2s motor, but that's about it. Probably best if you intend to race, and I'm sure you'll be wanting more torque after you're done with the novelty of having the 4.5 turn motor for bashing though. Getting a fast motor is unnecessary for racing. More often than not, you probably will turn better lap times with a turn or two higher motor.

I've read a 50/50 chance of the sensor port working well, or it just conking out, but who knows, maybe that issue is fixed with more recent VXLs.

If you have the VXL setup and can afford to run 3s, that's probably a far better option overall.

The VXL is a 10 turn motor btw.