what does "hit balance"mean?
“We should see how it feels.”
As she angled toward him, the doorbell chimed.
“Damn it. Hold that thought.”
He found Vinnie at the door, and realized he hadn’t hit balance when the sight of a cop, even an old friend, still made his heart lurch.

Question

what does "hit balance"mean?
“We should see how it feels.” 
As she angled toward him, the doorbell chimed. 
“Damn it. Hold that thought.” 
He found Vinnie at the door, and realized he hadn’t hit balance when the sight of a cop, even an old friend, still made his heart lurch.

Answer

I find this to be an unusual construction - at least for US English. I believe it means that the sight of Vinnie caused emotions he would not normally feel when in a relaxed state. In other words, his emotions became unbalanced. He lost is composure.

Answer

I've not heard of this idiom but it may be American.  From the context, it probably means that seeing a policeman makes him feel emotionally unbalanced. He probably had had a bad past experience with the police and he has not yet recovered from it.

So "hit balance" (informal) = "reach an emotional equilibrium" (very formal).