Verifying that there's no way to create a vCard from just the .abcdp files.

Those did used to be vCards. But, a couple iterations of the OS ago, they changed. I have not found a way to use them except as you found you can QuickLook them.

They are now XML Property Lists. The data is in there, but you would need something to extract the data into a vCard format.

Apple hasn't seemed to make Contacts able to read the Property Lists to import them.

If those .abcdp files are indeed XML property lists aka plists then in theory you could write a script to read data in from the .abcdp file and output as an equivalent vCard file.

Apple's defaults program will only read files with a .plist file extension but the plutil tool does not care about the file extension and of course if your script does it itself it can just look for things like <key>something</key>

I did not say it was going to be easy. :)

There is some stuff in there are one can see is relevant e.g. first name, Email, etc. it is mixed in with a lot of unhelpful stuff. You can use the following

plutil -convert xml1 sourcefile

and it will replace the contents with XML1 format i.e. plist format instead of binary format.

The hard part is scraping/parsing the .abcdp file, the easy part will be outputting as a .vcf (vCard) file.

Note: these files in their normal location of ~/Library/Application Support/AddressBook/Metadata/ are not accessible to command line tools unless you approve their access request in System Preferences -> Security & Privacy, you could and I would advise should copy them to a new location e.g. your Documents folder. Then not only have you got a backup but you avoid that security issue.

There are some libraries for python to help processing plists and you could then use that with a python script. Or you could brute force it - that is parse the file yourself in your script.

Getting back to a previous comment, there are some websites for posting 'bounties' to encourage developers to write you a solution for a (hopefully) modest fee. The following are not the one I was trying to remember but are similar, perhaps other people can suggest similar ones. See - http://www.koder.com/ or https://www.bountysource.com/

Mojave's Contact application appears to be using .abcdi metadata files. You can dump (not XML) these to the Terminal via:

plutil -p filename.abcdi

They contain nothing useful for a Vcard.

The *.abcdp files are stored in the ~/Library/Application Support/AddressBook/Sources/<source string>/Metadata folder — where I was not looking previously. Familiar with the Python xml modules.