Definitely looks like squash vine borer. If you pull it out and split stem and you will see the larva in there. I have had slight success slicing open stems, removing the worm, and throwing soil back over the plant stem so it will grow more roots. But what if there's more than one? So I generally snatch em out and plant some more. They're a real bitch though.

I actually lost a couple plants last week to them.