Yes, 1/8" stretch (if not more) over a 12' panel is definitely going to happen. Especially if using a softer vinyl. Heck, just the weight of it hanging against the wall will stretch it on it's own.
The key is to either design with that in mind (no continuous tight fitting elements all the way down the panel like repeating stripes, type, etc) or you hang your first panel slightly higher at the top and then let the material stretch a little as you work it towards the bottom.