I know full well that the A350 is the replacement of the A340s at LHR and the 747s at LGW and MAN but considering VS want to expand i think the 777x is a good option also if they want to serve high capacity routes if and when Heathrow expands. I don’t know, just a thought...
I think it's highly unlikely at this stage. The 777X is a very large twinjet and there's little VS need it to do that can't be done with what they have already, notably the A350-1000's. I know BA will operate both the A350-1000 and the 777X, but their needs are different and have the size and scale to accommodate these two types.
If VS need more aircraft in the coming years, I expect they will order more of what they have already.