Its too funny that this question is always asked.

You will not be billed for data on your phone when using WiFi. It is not possible for VZW to bill you for WiFi since you are not accessing their network. This is one reason why you cannot get picture messages over WiFi, because you are not on VZW network to access any data there.

Repeat: You will not be charged to use WiFi. Its just not technologically possible. You can search for this very question in almost any forum related to Verizon phones and find the same answers there