If you send and recieve texts through the google voice app, you will not be charged by verizon for texts. Since the app uses 3g data, verizon is unaware you are even sending/recieving texts through the app. Its just like browsing the web or streaming pandora (no extra charge beyond normal monthly data charges).
If you use the google voice app to make and recieve calls, it will still use your Verizon minutes (unless your google voice number is in your Friends & Family) because it uses your google voice number as a gateway to make/receive calls. For example, lets say you make a call to a Canadian number for ten minutes using google voice on 4/8/10. When you look at your April bill, you will see a ten minute call to your google voice number on that date.
When somebody calls your google voice number, google voice forwards the caller's phone number to your caller id, so it will show up as a call from that number (not your google voice number). Your bill will show a call from that number on the date you received the call. You do have the option to set google voice to forward your google voice number instead of the caller's actual number, but then you won't know who's calling. However, if you were to go that route and add your google voice number to your Friend's and Family, any call made to google voice would not be charged against your minutes. In addition, your bill would show a call received from your google voice number.
Please let me know if that's not clear enough or if you have any other questions...