HFS disks will not boot a 128K or 512K, nor can they be read on one without the HD20 INIT which is not supported in Mini vMac. So that message is to be expected.
Do you get the same message with the 400K MFS images?
Also why would you try to open these old disk images under Snow Leopard? What use would 25 year old files be under OS X? Also, it is a known limitation that MFS is not supported and HFS is read only. The act of opening them can introduce corruption that will prevent Mini vMac from reading them. Go back to your original untouched copies and try those.
Try using the Mac Plus build to manage your files as there are many problems the 128K system can introduce when trying to manage vintage files if you don't know what you're doing (and even then). You may have to transfer the files to a new blank disk image.
Do you get the same message with the 400K MFS images?
Also why would you try to open these old disk images under Snow Leopard? What use would 25 year old files be under OS X? Also, it is a known limitation that MFS is not supported and HFS is read only. The act of opening them can introduce corruption that will prevent Mini vMac from reading them. Go back to your original untouched copies and try those.
Try using the Mac Plus build to manage your files as there are many problems the 128K system can introduce when trying to manage vintage files if you don't know what you're doing (and even then). You may have to transfer the files to a new blank disk image.