It depends on authority and proof. It is not vigiantism if the area is private and the guard is given authority by the neighbourhood association (or whoever owns/is in charge of the land).
While the wealthy being above the law is a problem, the larger problem is the arbitrary nature of the law and the arbitrary nature of its enforcement. People wouldn't have to hire private guards if the law wasn't enforced. The police would enforce the law if there was less of it wasted on things that moral people do (like smoking pot).
Often the rich are happy to have the police not enforce the law and to do it with private security instead. Private security reports directly to their employers, the police have more public accountability.
Also reduced property taxes mean less police for everyone, so you get the "I only pay for what I need" anti-tax rationalization of those who live in gated and guarded neighborhoods which is ultimately socially destructive.
I agree with your sentiment regarding arbitrary law enforcement and the huge waste of times it is to chase people smoking pot and similarly harmless things. However, you seem to imply (please correct me if I'm wrong) that this justifies the vigilantism. I find that idea to be very problematic on many fronts.
Is it vigilantism for you to stop me from walking into your house? That is the crux of my argument. If you, or a group of people, rightfully own land then you should be free to pay guards to stop people to enter it.
While the wealthy being above the law is a problem, the larger problem is the arbitrary nature of the law and the arbitrary nature of its enforcement. People wouldn't have to hire private guards if the law wasn't enforced. The police would enforce the law if there was less of it wasted on things that moral people do (like smoking pot).