Watch the scene where Michael meets with Pentangeli at the old Corleone house in New York (the scene where Michael says "In my home! In my bedroom where my wife sleeps...") Michael explains that he wants Pentangeli to make a deal with the Rosatos as a means of deceiving Hyman Roth. Mike realizes that it was Roth who tried to kill him, but he doesn't want Roth to know that. He explains to Pentangeli that if Roth sees that Michael persuaded him (Pentangeli) to make a deal with the Rosatos, Roth would believe he still had a good relationship with Michael. This would make it easier for Michael to find out who betrayed him (it turned out to be Fredo, of course).

I don't think this situation is equivalent to Vito sending Luca Brasi to inform on Sollozzo (an incredibly unwise thing to do, by the way). Pentangeli was only telling the Rosatos he wanted to make a deal, not that he wanted to betray the Corleone family and join the Rosatos and Hyman Roth. Luca, on the other hand, was saying he was disillusioned with the Corleones and wanted to join Sollozzo and the Tattaglias. Furthermore, this was particularly unbelievable given that Luca was known to be Vito's most fanatically loyal person.