YYZ-ORD-TPE - AC/BR
from TPE up to you but - TPE to SGN (or HAN depending on where you want to go in vietnam. flight to SGN is on better 777)
From vietnam would fly SQ to SIN then SQ 787 to DPS
Lots of DPS to IST on TK 787
from IST several ways back - WAW/YYZ on LO, ZRH/YUL/YYZ or YTZ on TK/LX/AC, LIS/YYZ on TK/TP