Captain of the Guard is a title granted by the First Citizen of the 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 him full authority over the military might of the Vault City.
Obtaining this rank within Vault City will allow the player to go back to Citizen Sergeant Stark in the Corrections Center and demand that he pay Cassidy compensation for damaging his bar, which will earn the player $500 and 500 XP. It will also allow the player to bring Marcus and Lenny into the inner city without being stopped by the guards. Bringing Marcus to the Vault doctor will earn the player XP and a considerable amount of ammo, as he pulls years of accumulated bullets out of Marcus' body.
If you're willing to buckle up and walk the dialogue minefield, you'll need the following things:
- Speech more than 74% and a Charisma > 7.
- Be able to speak to Lynette, the First Citizen of Vault City, without killing her. No small task.
- Choose every response that addresses her as "First Citizen," including "Bye" responses.
- Stop the raiders, show Lynette the account book you got from the raiders and Bishop's holodisk detailing his affairs with NCR (located in John Bishop's personal safe in New Reno), and then deliver the disk to Roger Westin in NCR.
- When you get back, mission accomplished, if her respect for you is high enough, she awards you Captain of the Guard status.
- You will notice most of the Vault City citizen dialogues will change, especially Sergeant Stark, who will be none too pleased with your promotion. Take John Cassidy with you to Stark for more hi-jinks and a little more XP if you confront Stark on busting up Cassidy's bar.
The best way to insure Lynette's respect counter for you is high enough is to find a loop in the dialogue where you can continually address her as First Citizen - after becoming a Citizen, the best way to do it is to keep asking her about Vault 13 (if you haven't found it), choosing the response of: "First Citizen, it's very important to me that I find the location of my ancestor's Vault. If I may check the archives, I would be grateful." Loop it ten to fifteen times to be sure (you need at least a level 10 - I do it fifteen times to be sure), then just be sure not to anger her after that.