Remember that a 3G data stream contains both voice and Internet data, and the VPN encrypts only the Internet data. Therefore, your phone conversations would remain unprotected unless you use a service like VOIP. VPNs generally have strong AES-128/256 or similar encryption that will take much more than two hours to breach, therefore greatly increasing your security.
If the encryption is cracked, eavesdropping is most likely possible (think of those hackers looking at open wifi connections to sniff for passwords) and probably is a MITM as well.
However, you should still realise that spending two hours to crack a 3G connection is still quite a long time. Unless there is a specific reason why a person would like access to your phone, it is very unlikely that they will spend the effort to break the signal.