Normally you cannot stay in transit in Canada when traveling to the US as the US immigration and customs inspection is done in Canada prior to boarding the plane (this allows Canada-US planes to land at US airports without customs facilities). Unless YVR has a special arrangement for US arrivals staying in transit to US departures (and how many of those are there in a year?), you will have to clear Canadian customs in order to get to the US INS/Customs inspection and board your plane to the US. Maybe the RCMP can meet you and escort you to the US checkpoint without entering Canada, but I suspect it cannot be done. Luckily you have a few hours between flights so probably you will make the DFW flight, but be prepared for problems. Also the US security level is Orange now which means special processing goes into affect and your itinerary may attract attention (if for no other reason than entering the US twice within 24 hours). Why don't you go JFK-SFO-DFW instead and save some grief?