If the kernel has 802.1q support baked in, you don't need auxiliary software. Arch's wiki tells you how to do it using 'ip':
https://wiki.archlinux.org/index.php/VLANThis method should be compatible with rc.conf though they don't mention how in the wiki. You can set up netcfg profiles for starting vlans at boot, and examples on how to do that are in /etc/network.d/examples after installing netcfg. Or just make your own script in rc.d and add it to DAEMONS in rc.conf at the appropriate spot. Then it's just a matter of launching dhcpd on the interfaces you want it to respond on (which would be tied to a specific vlan). Utilizing the system's native capabilities is always a better choice than relying on third party software unless you really need to.