With due considerations to the recent developments in terms of PPTP based VPN connections that lead to compromise of data, usage of Wifi under a VPN is still highly secure for a number of reasons.
Firstly, if you're a casual user who's performing Internet Banking, the practical overhead of performing the exploit procedure and then the decryption of the captured data is quite significant. No local coffee shop hacker would do it, and professionals wouldn't be interested in spending his time on you.
If security is absolutely critical to your transactions then, there are a number of simple additions:
- Ensure that the VPN connection is at least an AES-256 with certificate authentication. Do the usual checks to ensure that the certificate is not compromised to a MiTM attack. Eg OpenVPN
- Keep the sessions inside your VPN connection them self encrypted. ie. Don't use HTTP, use certified HTTPS, replace FTP by SFTP, etc.
- VPNs are over-rated as per me. A good SSH tunnel configured properly works as a lower cost and light weight, easy to deploy solution for encryption. Plus you channel it through your rented server and if you have the need and resources you could for example, setup any custom modifications of the encryption procedure. I'm not encouraging obscurity, just saying you don't have to limit yourself to the current textbook compliance procedures and thus use infamous but more secure encryption techniques.
My point being, for banking, the chances of an attacker being able to break through a VPN encryption, then say a SSH tunnel and then finally your HTTPS web page traffic is extremely improbable, even for highly confidential EOW data.
And even then, banking sessions generally last for a few minutes at max. Apart from certain broken information, there would be no way in any practical banking system with even basic security to use that in order to steal money from you or hack you.