That cap that says vacuum on it most likely is your Pressure Vacuum Breaker (PVB) - which is your backflow device. It simply protects your water supply from contaminants. As Wetboots said, "apprentice" your service tech when he comes out to turn the system on. Depending upon how much knowledge you have in irrigation, you may be able to troubleshoot some problems yourself, others, you may have to call in a pro. If you are looking for more information about irrigation know how, check out www.irrigationtutorials.com
Tony Posey
Ridge Run Landscapes