You will "transform" into a character with less defensive capabilities but with more offensive skills. In order to use Vanguard (or the Vanguard related skills, not sure) you need a dualsword.
I think last 5 hours but you can transform and untransform any time you like with some seconds reuse.you don'thave to use duals but the skills you gain from the transformation need duals to be available.is not drop your defence us number from your stats but drop resists and Shields defence etc
u can run all your buffs and pet some buffs you will have to go back and forth to activate.
in vanguard form just think of glad powers without having to charge up to use em works great for fast lvling. provided you got a really nice set of dual's.