No. He did not descend into madness with the Vex. I think you are mixing that up with Osiris. Let me clear something if you don't mind. Toland the Shattered was a Warlock who was fascinated with the darkness and did everything in his power to investigate, going into the darkest places reachable and allowed by the Vanguard. According to a message said during the bounty and mentioned by Ikora Rey is, "Be careful Guardian. There was a reason Toland was cast out of the order." Suggesting that he may be alive but a roamer from planet to planet. Osiris was a Warlock who became fascinated with the Vex and did everything in his power to look over them before he went into the vault. From what I have read and deciphered, it is possible he made it as far as the Templar before he was destroyed, or he was 'marked for negation' by an Oracle. Kabr was a Titan who ventured into the vault(Supposedly by himself or with a party, unsure.) when he went into the vault, he found that the vex could not touch him. Now. This may sound wierd. But after looking over the Titan subclass 'defender' and the fact that Vex are part of the void. This part may have to do that Kabr was the founder of the Defender subclass and allowed future Titans to wield the Void as a defensive weapon. As he ventured into the Vault, it is not known but I believe he was killed by Atheon or the Gatekeeper. (Sidenote: It seems rather wierd but a potential hypothesis. With the vex numbers so great, it is possible that besides Atheon, that the gatekeeper(s) and The Templar may be able to be remade or re-inputted different roles and placed into the Vault. I.E The Templar dies and Atheon/whatever being controls the Vex remakes a new Templar and places him into the Vault) There. Schpeel offer. I'm going to go rip some Psion heads off.

He also could be like the guy who was corrupted by thorn