Yes, unions...and threaded ones in case those ever need changing! [For some reason I always refer to them as universals. I don't know why, as they are anything but universal] But, yes, I added one on the entry side (after the pipe shut off from the house, NOT the built-in one on the vacuum breaker unit) and then again after the shutoff on the exit side before the pipe went back into the ground and on to the valve box.
FWIW here in NC it is code that all irrigation backflows (vacuum breakers not allowed, must be dual RPZ) must have an insulated box around them. It is also required that the irrigation line from the city water meter to the backflow be 24" below ground. Seems a bit deep but whatever.