Thanks André,
I checked the descriptor file, and you are right. The problem is the two missing data files. I have been using Fusion for years now, in exactly the same way, and I am really puzzled how this could have occurred so suddenly. If it were user error, it would mean someone would intentionally have had to go in to the package, and delete them. Since this has not occurred, the only thing I can think is that the problem lies with Fusion.
I have searched around, and have not come up with anybody else experiencing this issue.
Since even my nightly backup is missing the data files, I am going to have to go to an offsite Time Machine backup, and see if I can recover my entire VM from there. At least, I will be losing only three months of work versus 18 months!
I will keep this post open for the moment, and update with how I went with the Time Machine backup.
John