Answer: Since I can't think of any insect that would tear off leaves by the stem, I suspect a critter (Deer or woodchuck could conceivably do that.) or other mechanical damage (A baseball, for instance.)
However, just to be sure it is not a borer attack, check the vine for holes and look for "sawdust" nearby. If there are borers, slit the stem lengthwise where the hole is and destroy the larvae. Then, cover the damaged part of the vine with moist soil to encourage it to root. Then, to try to prevent reinfestation next year, be sure to clean up, remove and destroy all the vine debris as soon as you have picked the pumpkins. You should also disturb the soil in the fall and turn it under in the spring to bury any remaining cocoons.
Good luck with your pumpkins!
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