The heart and lung shot works great...I do use it from time to time but I've gotten accustomed to using brain shots almost exclusively. It's a bit easier for me to visualize the location of the brain, regardless of what angle the squirrel presents to me.
https://www.youtube.com/watch?v=AA9nN_IkdfE
The vitals shot does yield a less animated expiration so I get that appeal but another reason I prefer the brain shot is it produces an instantaneous halting of consciousness and I view that as somewhat more humane.
Another thing is I'll often take a .177 cal on gray squirrel hunts and pesting sessions...something in the 10-15fpe range is a beautiful thing for being discreet in a suburban setting, and in the woods it often enables taking multiples back-to-back-to-back. Grays are notoriously tough critters though and this caliber and power level is perfect for brain shots, whereas I've had a few runners when taking vitals shots. I'm perfectly willing to admit that difference may be my fault. In .22 cal and above, I find either shot placement to be equally effective.