Chances are the LHR-VIE-LHR ticket is a cheapie with no refunds/rebookings, so if your flight to LHR is late you're SOL. Going the other way it's a bit safer, as UA will probably put you on the next available flight. It depends on your risk tolerance and how much time you leave between ...-LHR and LHR-VIE. I do this and usually come out ahead, but I try to leave plenty of connection time.