When this SIGNI attacks, if its first [Constant] ability causes the SIGNI in front of it to be banished, this SIGNI's attack crushes the opponent's Life Cloth, as Vier=Rikabuto has no SIGNI in front of it during the battle step.
If you have Vier=Rikabuto on your field and you use Dark Matter during your own Attack Phase to send Rikabuto and one of your opponent's SIGNI to the trash, Rikabuto's second [Constant] ability will activate. You add the top card of your deck to your Ener Zone.
Before movement abilities, [Constant] abilities are confirmed for their activation condition. Vier=Rikabuto is sent to the trash at the same time as the opponent's SIGNI. While in the trash, Rikabuto checks the conditions for her second [Constant] ability. While still in the trash, Rikabuto activates her second [Constant] ability.
While you have 2 Vier=Rikabuto in play, if the opponent has sent 1 of their SIGNI from play into the trash due to Iona, Ultima/Maiden's third [Constant] ability, and you send 1 Rikabuto to the trash, that Rikabuto's second [Constant] ability will still activate.