Ive been having the same problem, and i was told that it was vine borers however when i cut further up the split (like i was told to do) i found no sign of a borer inside, but ive tried both packing the part of the vines that were split with dirt (it was a desperate decision since the vines were suffering after i cut into the vine looking for borers) and burying the affected vine at least 3 inches up from the split, and between the two attempts i found that burying them worked the best, my pumpkin plant seemed to benifit greatly since it grew considerably over the next week after i buried it. As for an explanation as to why it split like that, ive never found a good explanation past it being due to vine borers. But try burying the stems like i said, i hope it helps!

That's really interesting! I'll try burying it.

I'm curious: When you saw the damage you attributed to Vine Borers, was there only the split in the vine or did you see brown sawdust-looking stuff too?

I had a borer problem previously and it looked messier, i guess. There was brown crumbly stuff. This time it really is a clean straight split. I'm attributing the damage this time to over-watering, both by myself and Mother Nature ;)

Thanks much - I'm gonna get to burying and remain optimistic!