Yes you can put vista mail into windows 7 as long as you have access to vista win mail directory. Did that on my wife's new built (I3-540 Win 7, 32 bit) a little over a month ago and she has had no problems. You have to first go in to vista and change permission so you have permission. (see if I can find the Link). Then copy to a thumb drive. On the Win 7, Rename win Mail directory to Old Win Mail, then copy Vista win mail director there. Don't forget if for Win 7 64 Bit, you need the Vista 64 Bit. Win Mail.
Here is one linkhttp://www.tombraiderforums.com/showthread.php?t=155240
Found my notes, here is a better outline of the procedures:http://www.sevenforums.com/tutorials/5481-windows-mail.html