From the VIM documentation:
- The algorithm used is breakable. A 4 character key in about one hour, a 6
character key in one day (on a Pentium 133 PC). This requires that you know
some text that must appear in the file. An expert can break it for any key.
When the text has been decrypted, this also means that the key can be
revealed, and other files encrypted with the same key can be decrypted.
If it was feasible on a Pentium 133, I think you have a very good chance. 🙂
I would either use #vim to ask for details on the algorithm or browse through the source code to figure out how the encryption is working and then write a cryptanalysis program.