I've always liked the "oblivious" look on vault boy's face, it added to the joke. In a lot of ways the obliviousness reflected into the unforgiving nature of the Fallout world. When I first saw the vault boy illustrations, I took them for what they were: Vault Tec 50's style illustrations on how to deal with the world around you. But then they've became much more interesting as they evolved with the outside world (using guns, etc.), as if some how the illustrator for Vault Tec was contracted to create these crazy new vault boys and retain his innocence.
I think Bethesda's choice to change his expression loses a bit of the joke. I love how he's kicking the pregnant woman without a care in the world on his face.