Do you have the Flukers sun dome or an 8" diameter dome? What I have found is you need to have a 10" dia. dome so the bulb does not get to hot. And the dome should me mounted to a spot that does not move on the viv. Ounce I got the big dome mounted on my viv and stopped using my x-10 system to turn on and off the light the MVB became reliable and lasted a year. I have my light pointing straight down, I really do not know if that makes a difference.
The MVB are a pain because the mfg. do not give enough info on how to use them. They cannot be treated like a regular spot light, but they do solve some problems like getting the UV light where you need it and can save money by not needing to be replace as often as a fluorescent tube. My viv is to tall to have a fluorescent tube within 10 inches of my animal other wise I might have gone the Reptisun route as they seem to be more consistent in uvb output.