Originally Posted by djjaguar64
Since you will be landing in YVR(first port of entry), you will have to go through Immigration and Customs procedures in YVR and then proceed onto YYZ. The flight from YVR to YYZ is a domestic flight and thus no processing needed in YYZ. It would have been the same if your onward journey was YVR and you were flying in from LHR to YYZ.(customs in this case will be in YYZ)
I think you mis-read the OP. He's flying LHR-YYZ-YVR. YYZ will be his first port of entry so he'll clear customs in YYZ and fly domestic to YVR.
mpc1