It depends what you want to be secure against. Using an encrypted volume for the VM makes it so that someone with physical access to your computer cannot see your data without the key - it provides local secrecy
. Using TOR makes it so the people you're connecting to cannot see your identity - it provides anonymity
. Neither of these inherently strengthens the other.
Actual security depends on other factors. If the web browser in your VM has a security vulnerability, the site you connect to could exploit it to disclose your IP address and upload a copy of all the files in the VM. If the VM has a vulnerability they may be able to view the rest of your computer as well. To protect against this you need to run the latest versions of each.
One good option is to use Tails
on a dedicated netbook with no hard drive, just a CD drive. Tails at least makes sure that all internet traffic goes through TOR (it firewalls everything else off) and that no logs are kept after you shut down, and using a dedicated netbook prevents access to your data, but it still can't prevent disclosing your IP if there is a vulnerability in the included software.