I am helping my friend with her garden here in New York City. We are trying to figure out what is eating her new sweet potato vine. These are growing in a sunny location next to her front stoop.

I'm guessing from the photo she took that it is a caterpillar-like critter doing the damage. The fact that the holes are in the middle of the leaf and not at the edges makes me steer away from slugs or snails. What do you think?

Any suggestions on remedies? Diatomaceous earth?