You may be the victim of squash vine borer. An olive brown moth will lay is eggs on the stems near the base of the plant. When the eggs hatch they bore their way inside the vines. It is a difficult pest to control. You can vigilantly spray the base of the plant with rotenone or pyrethrins to kill the larvae before they enter the vines. Cover the plants with floating row cover early in the season (you would need to hand pollinate the flowers). Another method is to wrap the base of the plant is panting hose to keep the larvae out. The problem is as the season progresses and the vines get longer the larvae enter at other sections of the vine. You can try to save infested vines by digging out the borer with a knife and heaping soil over the vines to induce rooting. Rotate your plantings each year.

Leave the pumpkins on the vine for as long as you can. Protect the bottom of your pumpkin from rot & insects by placing a liner such as a board, newspaper or landscape fabric. If you are forced to pick them, store them in a cool,(50-55) dark place. My pumpkins are doing the same thing and I have squash bug and vine borer-a double whammie. To see if you have squash vine borer split open one of the dying vines and check at the node (where leaf comes out) you’ll see a fat,cream colored larva.

It sounds as if your vine is much healthier than the yellowing vine. Barring disease and insects the yellowish vine may need more nutrients. Pumpkins are heavy feeders and your soil may have more nutrients. On the other hand, it may be an insect problem such as squash vine borer which will cause the plant to suddenly wilt and upon further inspection you would find a hole or chewed stem at the soil line.