As you are MFUing then you will need to call BA to book. You may be able to get them to waive the offline fee - I have had about 50% luck as a silver. If it was a regular booking you could do a multicity on ba.com with no extra fees.
You probably have to wait until June for a fare sale covering this time next year. If the dates you have in mind are important you may want to consider booking outside the sale (ugh!!) - or at least keep a very regular eye in MFU availability. YVR is a popular route and SFO availability seems to have gone from fantastic to at best average recently.