Captain of the Guard is a title granted by the First Citizen of Vault City to exemplary citizens that have gone above and beyond the call of duty to protect the city's interest and independence. In essence, it is the highest military rank one can hold, giving the citzen full authority over Vault City's military might.
Obtaining this rank within Vault City will allow the player character to go back to Citizen Sergeant Stark in the Corrections Center and demand that he pay John Cassidy compensation for damaging his bar, which will earn them $500 and 500 XP. It will also allow one to bring Marcus and Lenny into the inner city without being stopped by the guards. Bringing Marcus to the Vault doctor will earn the XP and a considerable amount of ammo, as he pulls years of accumulated bullets out of Marcus' body.
To be appointed Captain of the Guard, you must:
- Be a Vault City citizen
- Have a Speech skill above 74% and a Charisma > 7.
- Complete the Stop the raiders quest by showing Lynette the account book you got from the raiders and Bishop's holodisk detailing his affairs with NCR
- Have a high level of respect (See below)
- Complete Lynette's follow-up quest to deliver the disk to Roger Westin in NCR.
If all the conditions are met, you should receive your promotion after reporting back to Lynette after your delivery.
In order to be promoted, completing the quests isn't enough, you must also have a high level of respect from the First Citizen. Respect is gained by using her proper form of address (First Citizen), and lowered by talking to her when not a citizen, referring to her more casually, or otherwise upsetting her.