You kill Tenpenny in the mission End of the Line. As this is the last mission in the storyline, you cannot kill him any earlier. Also there is no San Andreas mission called Tower Quest, unless this is your own special name for one such as Breaking the Bank at Caligula's. But Tenpenny isn't in that one. Have you installed any mods?
I doubt s/he meant the GTA Tenpenny, seeing as this was in the catagory The Vault.