Network Anonymity is largely how much you perceive yourself to be anonymous. While using a VPN and a layered-browsing method might seem to make you incredibly anonymous, ultimately your connectivity is passing through uncontrolled territory and could be monitored.
Weaknesses of TOR can be found on the wikipedia site: http://en.wikipedia.org/wiki/Tor_(anonymity_network)#Weaknesses
TOR's weaknesses are varied and subject to attack from different vectors. Some involve traffic pattern analysis which can reveal nodes, looking at captured packet data after it exits the TOR (or in your case, the VPN endpoint enroute to it's final destination), and some protocols contain real IP information which could reveal you.
Your VPN, since it is routed through the TOR network, largely serves to conceal your traffic from anyone listening on the TOR network itself. Once it leaves the TOR to the VPN endpoint, some of the TOR attacks become viable. (They probably won't know what is IN the VPN traffic as it is encrypted.) When the traffic leaves the VPN endpoint to it's final destination, it may or may not be encrypted anymore (depending on the traffic), and that data could be used to find out more information. Also, the VPN provider, may keep logs about who and what accesses their network for whatever reason. They can say they don't (some paid ones say so), but you can't ever really be sure. That alone may be a compromise.
Either way, it is a lot of work to put all the pieces of the puzzle together, which is the primary objective in the first place.
EDIT: Oh, one more interesting thing I forgot to mention...
Let's say you had accessed a website with a userID and password before anonymizing yourself, and the connecting information get's logged (IP address, yadda yadda) then you connect to the same website AGAIN later with that same user name and password (but this time from the TOR/VPN combo) the owners/admins of the site could look up that information and identify you almost immediately if they had the inclination to do so.