So, I tried to monkey with a VMWare virtual disk with VirtualBox, and VirtualBox corrupted the VMDK file. After getting the error “This is not a virtual disk” in VMWare, I freaked out a bit and sought a solution. It turns out, it only garbled a bit of the descriptor file, and did not corrupt the actual data at all.
The fix is easy. The first file of the virtual disk is just a text configuration file. Mine had a garbled first line. I simply removed the garbled first line and replaced it with the first line of a known good VMDK descriptor:
# Disk DescriptorFile
Worked a peach.