Can you post the relevant /etc/rc lines?
vlan create is supposed to create one or more vlanids on an interface. vlan add will add a vlanid to an interface that already has vlanids configured.
The man page says that vlan create will convert an interface to a vlan only interface, making it sound like you must run vlan create before vlan add - but it's possible that the code will, if vlan add is run on an interface that hasn't had a vlan create run, do the necessary stuff to set the interface up for vlans.
I would change the system that does everything my vlan add to vlan create, just in case that "extra functionality" goes away in the future.
Bill