Interesting why they would do that.
For a full on emergency, Shannon or Dublin would be closer diversions.
J
Came across this thread while I was looking for something else. For those who aren't aware of me, I am a retired (2020) Virgin Atlantic Airbus Captain and previously RAF pilot. I have flown this very aircraft (G-VKSS) many times.
Diverting an aircraft with around 260 people on board is a major event, both for those on board and the receiving airfield. The A330-300, while not among the very largest aircraft, is extremely substantial and not easily accommodated by that many airports in UK. Unless the aircraft is about to fall out of the sky, or (in a medical event) our ground-based medical experts advise it, we will try to divert to somewhere on our network so that there is both engineering and passenger support. Out of Heathrow, that basically means returning to Heathrow.
An engine vibration alert is not a serious event, but taking a two-engined aircraft out over up to 8 or 9 hours worth of Atlantic Ocean, often three or more hours away from an emergency diversion airfield (the A330-300 now has 210-minute ETOPs clearance, I believe, while the A330-900 is 285-minute) is hardly the action of a safety-minded pilot. Turning round to Heathrow still leaves plenty of options should the vibration deteriorate, but remember that the aircraft is certified for single-engined flight over water for over 3 hours, so an engine shutdown, should it be needed, is not a disaster.