It's probably been mentioned, but you are wrong about the multiple attempts. The line "No one can open the Vault alone. I opened the Vault. There was no one with me but I was not alone." is in reference to the fact that everyone he went in with was killed by the Gorgons. The Vault itself is an attempt to actually change the fabric of space-time. The Gorgons are extremely powerful creatures that can literally "write" you out of time. All of Kabr's team (the ones that ventured far into the Vault with him anyway) were seen by the Gorgons and were erased from existence. Kabr doesn't remember them because they technically no longer exist. However, he knows that you CAN'T open the Vault with one person. The contradiction slowly is driving him insane. Edit; my bad, you already understood that.