Hi there,
It's Stuart here, thanks for posting on our forum.
We're looking .vhdx, however at the current stage only .vhd is supported within BackupAssist.
I'd recommend that you sign up for our newsletter as this will make sure that you're kept up to date with everything going on with improvements we're making.
You can sign up for this by visiting
https://www.backupassist.com/newsletter/.