iCal backup files (.icbu) contains backup of calendar data, including contacts. So it is likely that there may be a way to how you can extract the contacts from .icbu files and saved them in vCard file (.vcf). Either by recovering the backup, or by using some kind of conversion utility.
Convert Apple iCal backup to vCard contacts.
We have found one software suitable in our database for this conversion.
Apple macOS / Mac OS X software - convert icbu to vcf on OS X
Apple Calendar (iCal) is a personal calendar application bundled with Apple Mac OS X since 2002. Calendar was the first calendar application for Mac OS X to offer support for multiple calendars and the ability to publish/subscribe calendars to WebDAV server.
Convert from icbu file
to vcf file
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.