Dear Analyzer,
You can draw up to 100mA before the device is configured and up to 500mA after the configuration is complete. If You just want to power something from the USB than as far as I understand, up to 100mA.
I would suggest You not to use USB power as a reference first, because of the voltage drop, which can be worse than Your requirements and second, because the USB power may be noisy. If this is an analog reference, up-convert USB power to a higher voltage with switching mode converter and then filter it out with linear regulator.
Best Wishes,
F.S.