If you truly mean a gpg file, then the gnupg plugin as mentioned is your best option.
If you mean "how can I open, edit, and save an encrypted file with vim" then you might also explore the -x option. It will allow you to enter a password and then save the file in an encrypted form, and does the right thing in terms of encrypting the .swp file as well.
- the cm=blowfish option is vital, otherwise you're saving with very
- I don't know if vim scrubs the memory before exit
Also, I don't know if keepass (mentioned above) as a command-line counterpart for Linux, but PasswordSafe (available for Windows and Linux) does: http://nsd.dyndns.org/pwsafe/
It allows you to edit entries in the shell, though if you want more freeform notes, full-screen editing, etc. you'd want something vi-friendly.