I've been using mutt with gmail. It drops into vim when it's time to compose a message. I've never heard of vmail. It sounds interesting, but I would personally recommend mutt over vmail. mutt is quite vimish.
The first thing I see on that page is:
set imap_pass = 'yourpassword'
I get scared and run away...
You don't have to store your password in plain text just because it gives you the opportunity to do so - I use mutt without a stored password for IMAP. It means that I have to enter my password between one and three times per day, but that's a minor inconvenience to me.
More seriously, I thought of it as (1) a cool hack and (2) akin to Zawinski's law[3] about the way that software expands to consume more tasks. More serious Vim hackers might object that it amounts to the Emacs-ification of Vim. (Given the number of plugins to add calendars, todo lists and Org-mode to Vim, I think that battle may be lost.)
[1] http://danielchoi.com/software/vmail.html
[2] https://github.com/danchoi/twim
[3] http://www.catb.org/jargon/html/Z/Zawinskis-Law.html