For what I read (again, remember I'm far from an expert), Tor obfuscates your entry point to the network, but the exit node can still be used to track you. Adding a VPN is said to close the circle.
Tor obfuscates your entry and exit point to the network. Your exit node can only track you if they are also controlling your entry node and can link your entry and exit traffic between the two nodes, effectively mounting a Sybil attack.
If this is a concern of yours, then you could consider connecting to your VPN first, and then connecting from your VPN to Tor, effectively using your VPN server as an entry node in to the Tor network. If your Tor nodes are malicious, then they could only track you back to your VPN server, and not to your real IP address.
You should never connect to Tor first and then to your VPN, since that defeats the entire point of using Tor. All your anonymized traffic which has been passed through various Tor nodes and circuits would then all pass through a single VPN server and therefore all be linked back together.