Hey i just purchased an Accel mastemind monitor off ebay. It was made by bully dog for Accel. My question is how does it measure boost pressure if it doesn't have a vaccum hose?

it reads via the can bus and built in boost sensor

So it something that the Accel monitor does to read it. I know the computer on the truck it self doesn't monitor boost pressure.

yes the truck continuously monitors the boost PSI. your monitor just grabs that reading and displays it.