The vans are non-transporting intercept units.
Cambridge started with a decent idea, and then screwed it up.
I believe they started with BLS non-transporting response by CFD with an ALS unit from Pro. Ok, no big deal, lots of places do it. CFD got the run numbers they wanted but didn't have to get their hands too dirty. The main EMS work was done, granted, by a private, but at least you'd be taken to the hospital by someone who probably wanted to be there (its also worth noting that Pro is CAAS accredited, a process not many services volunteer to put themselves through) .
Nowadays, a 911 call could conceivably receive a response of an engine, medic van, and Pro ambulance. Probably the cops too depending what it is. I don't know for sure what prompted that change, but I can't imagine it's been some miracle of efficiency in patient care.