It depends on what booking code you were booked on for YVRSEA.
Usually, International J gets booked Y on the domestic sectors when you book on AC.com, which gives you latitude.
However, I've seen aggragators like Expedia book that part in Tango K, effectively selling you YVRSEA in Tango as one way, and then SEAFRAGVA on the "J" fare because it's cheaper.