Actually, I do have to agree that the distance from a typical VX gate to a typical UA gate is indeed quite far. The Int'l terminal itself is quite long and takes a few minutes to traverse, and if you're departing from one of the remote UA gates (past the RCC), add on another 5 minutes or so. And then there's T3 security.

Assuming you take the AirTrain, I'd allow 15 minutes transit time minimum, THEN add on whatever time you will need at security (less if the elite security line is open, more if it's not).

I'd only chance this 50-minute connection if I were on the same ticket. If (as I'm guessing) you're on separate tickets, you'll likely make it only if you have your BP ahead of time and the VX flight is on time. With the luck I've been having lately (25-30% of my flights being significantly delayed), I would do the 3-hour layover and spend the time in the lounge or in one of the great restaurants in the Int'l terminal.