Have a look here, click here.
Create a destination folder on the Vista desktop and export to there. Transfer to the Win 8.1 laptop via usb flash drive and use windows live mail to import from the folder.
Logging on to Outlook.com using your Hotmail or Live accounts should also bring up all your folders from Windows Mail.
You can also synch all your accounts using Windows Live Mail on the desktop on your Win 8.1 laptop. Much easier than using the Mail app.