Light and Motion addresses the issue in their FAQ page: http://www.bikelights.com/Support/Faqs.htm

Here's what they have to say about the issue:

Why doesn’t my wireless computer or HRM work when my ARC HID light is on?
The ARC HID light produces a weak EMI (Electromagnetic Interference) that interferes with most wireless transmitters on cycle computers and heart rate monitors. You will need to either move the computer away from the light, approximately 12”, use a hard wired computer, or ride without that annoying training device and just enjoy the night!

Moving the light and the computer/HRM away from each other has worked for me. Or if you use any model other than the Vega, you can use the cabeza mount to attach the light to your helmet and that will do the trick as well. Usually when I ride at night I like to pay attention to my surroundings and enjoy that cool feeling of "people must think I'm crazy but this is sooo much fun and they don't know what they're missing." So I just click my computer out of its mount, toss it in my jersey pocket, and check the data *after* I've enjoyed my ride.

Hope this helps,

~Cari