I'm not an expert on the issues / benefits of mounting the SD card as C:\SDCard?
But since you're mapping the VHDX to a new drive letter anyway, what happens if you don't mount the SD card into C:\SDCard, and try to mount the VHDX from the SD card's original drive letter?
Even if it isn't what you want to do in the long term, it may help find a path to the solution.
So if your SD card was drive F:, for example, then instead of
select vdisk file="C:\SDCard\MyStuff.vhdx"
Incidentally I had a tablet which wouldn't upgrade to Windows 10 build 1511 unless I removed the SD card.
select vdisk file="F:\MyStuff.vhdx"