If you want email to have autocomplete, then it does. 'Email' isn't a client. The client you choose to use is. Write your emails in vim -> now you have autocomplete.
No. The way to write your emails in vim is to use an email client that lets you open the text in an editor of your choice. E.g. mutt, or a webclient combined with an add on that let's your open textarea in an external editor (vimperator or its all text).
P.s. emacs sounds like a great OS - are there any add ons that allow one to open its textareas in a decent editor? :p