Well it isn't ideal, but it won't kill the plant. Think about it this way, that plant is now less susceptible to the squash vine borer. The borer would have a hard time getting to the inside of the vine, now that the insides are on the outside!

Based on your zone and the length of the stem below where the cotyledons were I am guessing that was a transplant, correct? As you can see the split occurred below the where the seed leaves were. In the future if you have a leggy seedling plant it to just below the seed leaves, that way there isn't a long potion of stem like that that can be stressed and split. Those splits ofter occur below the seed leaves. Direct seeding would be your best bet to avoid that in the future , but I realize you might not have that option. Honestly though it isn't that big of a deal it probably won't even affect the growth of the plant.

One thing you can do to help condition is mound up soil over split up to set of first leaves and water in good.