I've read a few posts about this new command available from agent version 14.5 and they suggest that you cannot update contacts.
According to posts here the below is what you can do with it.
Format of the Script is:
import_contacts_vcf <filePath> 1/0
0 = Add contacts to existing contacts
1 = Replace existing contacts with the new contacts of the vcf file
After some testing I've verified you CAN update contacts (using option 0) as long as your VCF entries have a unique identifier (UID), see example below. If you change any values other than the UID on a newer file it will update the contact with the corresponding UID.
BEGIN:VCARD
VERSION:3.0
N:_Name;;;
FN:_FullName Multi
ORG:Company;Department
TEL;WORK:+449999999999
UID:9420ba46-7e64-4d72-b6ea-25e102697764
ADR;TYPE=work:;;;City;;;
END:VCARD
Hope this is helpful.