VPK is 4-5 year olds right? Well kids that age do not always have their emotions under control and they are not yet able to fully grasp the conceot of possibly SERIOUSLY injuring someone.
My DD almost strangled another child a few weeks ago - not out of anger or because they were fighting, but they were playing dogs and she had tied a "leash" around her neck. I felt terrible about did, so did DD once she understood that she did really hurt her friend and could have really, really injured her.... but as I said for many situations kids that age cannot anticipate consequences of such magnitude yet.
No, you do not talk to the mom. The school is not even supposed to tell you who the child is that did it, but I suppose your child can tell you.
Ask the teacher what they are doing to prevent any further incidences, are they monitoring this kid, is this a recurring issue or out of character... and stay in conversation with your son about the incident and how it is important that he notifies a teacher if someone threatens to harm him or bullies him.
In our case we were notified of the incident and at home we talked to our child about the danger of what she had done, about our expectations and rules and the school has also taken measures by no longer allowing kids to play "pets".
Good luck.