Say I first connect to a VPN, and then open TBB. I think this is count as Tor oven VPN. Now, if my VPN connection (or my internet connection totally) drops when I'm working with Tor, will this unmask me to my ISP and/or to the website which I was visiting? In other words, does these kind of connection dropping lead to failure in Tor mechanism?
As long as you're using TBB as intended for browsing, there's no risk of your ISP seeing what sites you're visiting. There's also no risk of sites that you're visiting seeing your ISP-assigned IP address.
At the worst, your ISP will see that you're using Tor. That is, if your normal Internet connection is restored when the VPN connection drops, the Tor client will happily carry on without it. To prevent that, you can use firewall rules and routing to permit Internet connections only through the VPN service.
No, you won't be unmasked. There are two ways of developing of the situation when your VPN is dead, and it depends on how Tor uses your VPN :
- VPN adds default route So when the route fails, your Tor instance will just re-connect to the network(Tor network) and you will see just a small lag
- VPN is a proxy for Tor So when the upstream proxy server fails, Tor should stop working and you will see "no connection" page
But in both cases all your communications with Tor are in your private AND protected segment(localhost or LAN), so ISP is technically unable to see your queries to Tor, because Tor always stays alive and handles them. That's it! =)