Have done it with Virgin Premium Economy, a long flight at 10h 45min ish and at the moment Virgin are upgrading the IFE system. Our 747 hadn't been updated and it was crap!!!!
I have also done it with United, once via Newark & once via Houston.
If I was to do it again then I would do United via a USA city. I booked economy seats & then upgraded to their economy plus. Same service but extra leg room which is worth it. A small price to pay for comfort.
By stopping off then you get to have a walk around, stretch your legs etv whilst clearing US Customs. Also means that when you get to Vegas then you have no Customs as it is an internal flight.
Would agree with the 2.5hrs transfer. The time soon flies by the time you land, taxy in, get off plane, go through customs, pick up your case to transfer, find your new gate etc.
Enjoy your trip. Jealous!!!!