not an individual piece of text, no. but you can integrate whole file encryption if need be
http://vim.wikia.com/wiki/Encryption
In terms of more targetted solutions, I recently built a solution where the password was spat through gpg and then inserted into a file, so that did put us in a situation where the document containing all the passwords was itself readable and non-sensitive, yet in order to use a password you needed to cut out the relevant piece of text and decode with a gpg key, to which access, and it's passphrase, was controlled. This was in a dokuwiki though, so the user experience was always going to be much nicer than accessing it through vi.
