From your examples above, I'd hope you meant "where mount_lfs grabs a vnd number ..."
To grab afresh vnd is an easy part since vnd is now cloning psuedo device. Problem will arise when umount/vnconfig -u to identify which. And I've started inclining toward (mis-)using proplib like Xatom/Xproperty handling in Xserver (lalala forbidden territory, I know it) Toru Nishimura / ALKYL Technology