You were right if a new plant was to be installed, Here Coure and Vigo are assembled on the single assembly line. Plus for the Forturner they just need to change the body, The chasis imported is similar, And Since Vigos body is also probably iimported as CKD, So just importing price for the fortuners body might go up.
I doubt, If toyota sold a hand full of Fortuners they imported. But looking at the size of Fortuner if they manage to build it upto japanese standards, Pakistan has good market for it.
Another reasons for Fortuners sales might be the "The Vigo's Wadera image" Not many families buy it with the concept associated, "Vigo lee to koi peechei per jaega". Altho Vigo is doing okayish with sales (selling as expected by Indus)