If you have a return booked on the same award just call VX and let them know because of ~insert almost any reason here~ you won't be taking the final leg to LAX. Your return flight will remain ok. If you're checking bags ask the teammate at the counter to short check them to SFO. If you don't have a return booked on the same reservation then letting someone know you don't intend on taking the SFO-LAX leg is a nice courtesy as it keeps the gate agents from paging you and waiting till the last second to close the doors on an otherwise fully boarded flight.