Regarding the documentation, there are a few misleading examples. This is related to "Funtoo LinuxFL-2937 Vlan's stopped working" https://bugs.funtoo.org/browse/FL-2937

Both the 'trunk=' and 'vlan=' parameters seem to be required, and they must match the 'net.eth0' or 'net.whatever' name used for the initial interface-noip template. Otherwise you get the ambiguous error "* ERROR: net.eth1.1500 needs service(s) eth1" (specific to my case, the exact message will depend on the way the scripts were named along with the desired interface). I needed to add 'trunk=net.eth1' instead of 'eth1' as suggested by the web doc. I am not positive that vlan=1500 was completely necessary, but I added it anyway.

Also, on the same page but unrelated, there are two directives 'ipaddr' and 'ipaddrs' – it is not clear to me what the difference is, perhaps you could clarify.