We did 20+ Ba segments last year and I enjoy BA, however that said having to pay for seat selection for an already expensive ticket causes me pause.
LX flies a 340 to SFO. It has old First class seating but a nice new Business class product on most of the 340's. For 200 more pounds I would fly LX and stay with your FFP.
As for VS In many ways I prefer their business class to BA's. IN VS business class you are their best customer, On BA you come behind first class... does that make sense? perhaps not,for most people, but I always feel also ran when I do not fly in the top cabin. (insecurity I guess).
HOWEVER I would fly LX to SFO in C if I could have a new flat seat.