Using Vim as a password manager seems to be popular, but the built-in encryption options are limited. If you look at
:help 'cryptmethod' you'll see options for zip and blowfish, with zip as the default.
Stephen Braithwaite asked on the Vim development list if AES support was desired:
I am a programmer, and would like to add AES to available encryptions, which currently is mostly only blowfish. Would such an addition be welcome? Would such a patch contributed by me be acceptable? Or are there good reasons why Vim should have only blowfish, and the original encryption available in Vim which is known to be broken, and whose name I have forgotten?
Bram Moolenaar responded indicating more encryption methods are welcome, but naturally the patch would require careful review. I think it would be cool to have AES with Vim, so let's see if anyone takes up the challenge!