How might a major magnitude quake along San Andres Fault affect Vancouver?

The effect would be remarkably small. Even a large quake would be barely registered by a resident of either Vancouver, though a seismograph located there (and there are lots in both places) would certainly show it. Vancouver, BC is a little further away so it would feel a San Andreas quake less. The complicated geology and pattern of plates in the western part of North America mean that earthquake waves are rapidly attenuated over distance. The big one in either city is more likely to be caused by the Juan de Fuca plate and the Cascadia subduction zone off the coast of Oregon, Washington, and BC. Both cities, along with every place from southern Oregon north to Prince Rupert, BC would also feel it. It might also set off a huge tsunami.