No, it is not okay to stand on the porch and fire at random people in the neighborhood. It IS okay to fire on people trespassing on your land at 4:30 AM, ESPECIALLY if they're having a shootout.
Hey, i'm feeling generous. One night only -- I'll even grant you that it's A-OK to execute criminals in your front lawn, assuming you know that's what they are. And I'll grant that he knew. So, you're safe.
Nonetheless, I still have a problem with people firing shotguns into the dark. What if a kid was nearby? What if one of the guys was trying to talk the shooters out of it? What if your parents were driving by?
Even granting your conditions, I still don't agree that vigilanteism is the answer. So it went right this time. What if he'd killed someone you know?
That argument can be applied to anything. By that logic, the police should never be allowed to fire their weapons in public, because they might hit someone nearby. Responsible people can assess the risk and decide whether it is safe to fire on the people posing a threat to them.
So now that you consider his actions vigilantism, what would you propose he do? Call for help, wait while some idiot 911 dispatcher decides if the call is a prank or not, and then quietly wait in the corner with his wife while the cops show up 10 minutes later? And in the interim, what if one of the bullets comes through the window and hits his wife?
Look, the Supreme Court has ruled that the police have no affirmative obligation to protect you. You are on your own when criminals are in your midst.