What are these on the side of a vacuum breaker?

I had a similar device, actually a Zurn backflow preventor, on my house

(well that was before my caretaker failed to blow it out for the winter & insulate it....the unit froze & the housing cracked)

I'm pretty sure those little bleed / air inlet valves are for field testing the units operation. I would suggest tracking down the mfr's installation guide & the maint guide

I "suppose" you could use those ports to blow out the water but I'd be concerned about damaging the guts.

Since you got the unit with unions how about just pulling it out for the winter?

Or install a Tee in the supply line somewhere near the valve that shuts off the supply water to the sprinkler system. At the Tee place a ball valve with an air hose type quick disconnect for future line purging.

That's what I did, instead of having to remove a cap or plug and install a male q/d (& then reverse the process) every year....yeah, it was a bit of work but now the blow out process takes a few minutes & I have now connections to break and make.

Cheers Bob

those are the test ports used to make sure the valve does what it's supposed to do.

Backflow Preventer Test Kit - $489.00