It might offer you some ideas for starters. I have used Mailstore in the past to move large email archives in different formats between PCs. It worked very well indeed for this. Sounds though like you might be wanting to migrate between 'live' webmail on VM/GMail.
One of the email experts on here may be able to confirm whether the free version of Mailstore is suitable or not for that particular task and/or suggest alternatives to do this.