If you know the dates you will be travelling on your China domestic flights, you can purchase them as part of your international ticket.
It is best to deal with an asian travel agent in Canada before you leave.
For example:
YVR-PEK-PVG-PEK-YVR is cheaper than YVR-PEK-YVR + PEK-PVG-PEK
Also, some airlines offer "side trip addons" (available only through certain travel agents) that you can purchase in conjunction with your international ticket.
For example: If you purchase a CX YVR-HKG-YVR fare, you can purchase a HKG-PEK-HKG addon (which must be issued on the same ticket) for around CAD 300.
You can also consider buying a YVR-PEK-HKG-PEK-YVR (Basically YVR-HKG ticket with a stopover in PEK) published fare. (This will cost less than YVR-PEK-YVR + PEK-HKG-PEK).