Many articles helped me get here, but mostly good old SE!
HowTo Automatically mount and umount VHD files on boot/shutdown
First, I should say that I am mounting a VHD stored on a Linux File Server, mounting in over SMB.
Second, very important, is that if you don't detach the disk upon reboot, then when you try to re-attach it, it says "The file is in use and can't be accessed"..
So we need both automount and unmount. The unmount proved a bit tricker, since I cant just put it in startup folder. You have to use Group Policy. Also, when run as a "shutdown/startup" script, it ran to early, you need to run it at "Logon/Logoff"
I have 4 files in C:\mount
Now in start type/run `gpedit.msc` and add the scripts to logon/logoff