http://translate.google.com/#la|en|quid%20pro%20quo
Edit: and reverse-engineering from the english -> latin translator, we can finish the phrase:
http://translate.google.com/#la|en|quid%20pro%20quo%20Pro%20...
I don't know whether or not "quid pro quo Pro Quo manet" is actually sensible Latin or not, but something tells me that "What Happens in Vegas Stays in Vegas" isn't the most accurate translation.