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.