Pihole doesn't know the hostnames of the clients. You could set up conditional forwarding in pihole to your DHCP server. It (should) know the hostnames of your clients. With 5 VLANs you have to configure it manually via config files in /etc/dnsmasq.d/ as at the moment pihole let you enter only one subnet via GUI. But support for more than one CF field in the GUI is in the pipeline.
Or you could create "Local DNS Records" for all your clients in pihole.