This arrangement shouldn't be a problem. As long as the vines can reach the ground. They send out rootlets along the vines. My brother had pumpkins a couple years ago that wandered out of the cultivated patch. The vines that grew into the lawn performed better than those in the patch! He just mowed around the vines, and pulled at the grass around the vines. One thing I don't understand though: If you are avoiding cutting the sod out of your yard to plant a pumpkin plant, setting a pot on the lawn will probably kill the grass anyway. If you want to use a pot anyway, make sure you use a large one for the main root mass, 5 gallons or more. Once the vines start trailing into the grass, just leave 'em alone and mow around them. Also, another tip: if you are having trouble getting a vine fruit like pumpkins to produce fruit, you can artificially pollinate them (there probably is a technical term for this). All you have to do is identify a female flower(these will have greenish nodes between the flower and the stem. Male flowers won't have these.) You can either rub a qtip on the male stamens, and then on the female flower or just gently pull off the male flower and rub it all over the female flower. I do it this way, since it seems to me that the plant produces more flowers this way. More flowers = more fruit.

Tim