This technique even works with pitched tents, which is typically when you figure out that the zipper needs fixing. It works on luggage, backpacks, any zipper that has a metal slider.

If you overdo it with the locking pliers the zipper might get hard to zip. You can open the gaps on both sides in the zipper slider a little with a thin flat blade screwdriver, but it's best to work slowly and avoid that scenario.