The VS website is saying that the LAS flight operates on a 346 until 1/9 then it goes to a 747.
I suspect there is a short time where a spare A340-600 between the B787's coming online and the A340's leaving, hence the jaunt up to MAN.
I would expect the aircraft to position up the night before (morning slots are like gold dust at LHR to use on positioners), and position back down on Wednesday evening.