I have several thoughts on your situation:

1) AC via YVR to PEK is very unpleasent IMO because YVR has long lines and those 767s are slow, old, and cramped.
2) You might try routing via YYZ instead. You'll save a lot of time and ride on a newer Airbus. I've heard that YYZ-PEK has been pretty full during the summer, but would guess that things will calm down in the fall. The fact that it only flies several times a week may have kept it off of your radar. It would be worth your while to inquire further.
3) Fall fares are typically a lot cheaper than summer fares, but since it is still summer, I haven't noticed a big drop off..... yet.
4) I just priced BOS-PEK last week for August travel and got quotes for around $1000 on both UA and NW (I ended up going with UA on a different routing).
5) I suggest you wait till the end of the month before jumping on anything. Based on the flights I usually take (mostly UA SFO-PVG), loads have been light this summer. Therefore, I don't think availability should be much of a problem, if you are flexible. In other words, as long as you're reasonably sure that cheaper inventories (e.g. q and v on UA) will be available around your ideal dates, there is no advantage to booking now, assuming fall fares are less than or equal to summer fares.