Your wyvern's support job will be considered fully leveled , and as such will gain all the job's traits up to this level even if your own sub is underleveled.
This effect is triggered when Call Wyvern is used; simply equipping the Wyrm Mail does not add any traits to your wyvern. However, entering a new zone while the armor is equipped will add subjob traits to the wyvern in the new zone.
The wyvern will keep the traits until it dies, is Dismissed, or you change areas without the wyrm mail equipped. Unequipping the Wyrm Mail will not remove the traits.