On a linux DHCP server, you can add the following linke to dhcpd.conf file to display the vendor-class-identifier of DHCP clients:

set vendor-string = option vendor-class-identifier;

I added this towards the top of my configuration in both version 3 and version 4 DHCP server installations. I did notice that not all clients report the information back. For instance, some Wyse clients. You find the information in the dhcpd.leases database file on the server.