I play a Vylon Disigma and equipped it with Trishula (Water). Next turn my enemy summoned a Water Monster and attack my Vylon Disigma...he say to me that the rulings changed...and the effect of Vylon Disigma only works if he attack and not if he was attack...

Is that right...?

There aren't any rulings currently for Vylon Disigma. And judging by the effect it activates as long as the card battles with a monster of the attribute you have equipped on Vylon. That means it will activate when you attack or your opponent attacks Vylon. Vylon is like Catastor and obviously your opponent lied to you.

Thank you...then my enemy had try to cheat... ;D

Yea, unless the card says "When this card attacks an opponents monster" it will 99% of the time mean just battles.