A Valuable Lesson

 is a quest in Fallout: New Vegas.

Quick walkthrough
After giving Festus at least 51 Sunset Sarsaparilla star bottle caps and listening to the Sunset Sarsaparilla story you can ask Festus for the real prize to which he sends you to the vault. The way is blocked so you have to go to the first floor and to the West of that floor you can find a room with a safe. Under that safe is a hole through which you must go to reach the door to the vault. Inside lies your reward.

Detailed walkthrough
After finding at least 51 Sunset Sarsaparilla Star Bottle Caps the player can receive his prize from Festus which is, amusingly, the honor of listening to the real story of how Sunset Sarsaparilla came to be. Also, any star caps over 51 are simply converted into Bottle caps and given back to you. If you read between the lines you can see that the man who "invented" the drink actually had the true inventor killed and stole the invention from him. If you ask about the real prize he tells you that due to legal constraints the main prize had been removed and another is offered that should be enough. This triggers the quest A Valuable Lesson which instructs you to go to the Sunset Sarsaparilla vault to collect your prize. Access is made through the top floor through a hole in front of a safe and leads to a lower room which is filled with Sunset Sarsaparilla Deputy Badges.

Inside that room the prize is about 1300 Bottle caps (over 100 in each of 13 crates) and a total of 311 Sunset Sarsaparilla Deputy Badges (weightless and worthless - if you can't find all use the Z key to pull the crates off the top shelves). Also, in the corner we can find Allen Marks, a holotape and Pew Pew, a unique laser pistol. If you move him you can also take his Sunset Sarsaparilla Deputy Badge which simply falls off his chest. The holotape reveals that he was a murderer but does not reveal anything about how he got there. He apparently shot himself in the head to prevent death by suffocation as the vault was completely airtight. For more details about him and his demise see Allen Marks.

After finishing the quest you can no longer talk to Festus or play his card game.