Guild Wars 2

I just was wondering if the trait rending strikes (33% to cause vulnerability on crit hit) of the warrior stack % in some way with a superior sigil of frailty (30% to cause vulnerability on crit hit) or only the major % is applied. I know that 2 equal sigils stack % multiplicatively, but i dont know how and if it works also with sigils/traits. anyone figured this out?

I believe you would have, technically, 2 rolls on each hit. One roll with a 33% chance to cause vunlerability, and another roll with a 30% chance