How Do I Export vCards from Apple Contacts using AppleScript?

Sounds like a timing issue.

With this large number of transactions, the only way to debug is to run the AppleScript in a script editor, preferably Script Debugger 7.

I would add a log statement just before each card to identify which card it is failing on.

tell application "Contacts"
  repeat with cardPerson in people
    set nameOfvCard to name of cardPerson & ".vcf"

    log "Card: " & nameOfvCard   ### ADD THIS LINE

    set outFile to (open for access file (vPath & ":" & nameOfvCard) with write permission)
    write (vcard of cardPerson as text) to outFile
    close access outFile
  end repeat
  quit
end tell