Having just finished "Shattered Sword" (finally) I thumbed through it again, and my take is that if VB-8 had either followed VT-8 or just headed Southwest it's quite possible that Hiryu would not have survived the attack. According to Parshall and Tully, Kido Butai was stretched out in a rough line abreast formation with Kaga, Akagi, Hiryu and Soryu running from SW to NE. It's totally possible (to me) that resources could have been devoted to putting down Hiryu in the same attack. All it would have taken was a couple of hits and Yorktown lives to fight another day.
On Yorktown however, I agree with Oldcrow, she'd have needed a Major refit, probably in the states but would have been a pretty valuable asset in the later stages of the Guadalcanal Campaign, and as you (oc63) say, she may not have survived 1942 anyway, just another 'what if' that we all love.