I received the email, licensed messiah to a vhd and it works without a hitch. This saves me a usb stick/port and the danger of losing the license due to hardware failure. 

Whether vhd works or not can be easily verified by running the messiah license utility on the vhd, the same id is returned everytime, even on a winxp pc with a third-party mounter.

Of course go with a usb stick if you are unsure, it is just an alternative for more adventurous folks.