Why Can't We Be Friends?

Why Can't We Be Friends? is a side quest

Detailed walkthrough
Speak with Samuel Cooke and/or Philip Lem in the overseer's offices in Vault 19 to receive the two quest options - the offices are located on either side of the vault entrance. Then travel through either the Red or Blue sector living quarters of Vault 19. There are a few frag mines and fire geckos the Courier has to worry about upon entrance. From the living quarters, find the common room with the pool tables and a hole in the floor. It will bring the Courier down to the Sulfur Mines. The player may either choose to kill all of the fire geckos for Samuel Cooke, seal the caves for Philip Lem, or blow up both the caves and Vault 19.

Samuel Cooke
Once the player has killed all the fire geckos, go back to Samuel Cooke to receive some Powder Ganger fame and caps. He then sends the player character to the Red Rock Canyon to meet with the Great Khans, where the player has to convince Papa Khan to join forces with the Powder Gangers in the vault. After the player has convinced Papa Khan, return to Samuel Cooke to finish the quest.

Philip Lem
In the caves, follow the path to the right into a large cave. To the left is a large yellow rock formation peppered with lumps of gray rock. This is the “Microcline Rock”. If the player activates it, the rock contains enough potassium to create an explosion large enough to block the caves or destroy the vault. The player character will need a Science of 40 or more to receive the options. Choose to "Plant enough C4 to blow up the caves" (Explosives skill of 60 and 3 C-4 plastic explosives). Return to Philip Lem and let him surrender the gang to the NCR to finish the quest. Alternatively, with a Speech of 40 or more, the player can convince Philip Lem to join the Great Khans, although the speech check is not required if the Courier has a "liked" reputation among Great Khans. Philip requests that the Courier acts as a liaison and requires them to visit Red Rock Canyon and talk to Papa Khan to secure their alliance.

C-4
Here the player can find four packs of C-4 within the vault:
 * Between filing cabinets in the clinic, behind the front desk.
 * Located between the two vending machines in the cafeteria.
 * Outside the underground entrance to the overseer's office elevator near the doorway with the fire hose box. When facing the fire hose box turn left towards the junk pile, the C4 is on the left side of the junk pile.
 * Located under a couch in the Blue Sector living quarters (not the first floor). Go to the second room on the right, then to the couch left of the door. The C-4 should be next to the right front leg of the couch (looking at it; left leg sitting in it). However, it may be completely invisible or disappear when the player approaches it. It can easily be found by crouching and inspecting the floor close to the leg.

Blow up Vault 19
In the caves, find the Microcline Rock and activate it. Choose to "Plant enough C4 to destroy the caves and the vault then run like hell." (requires an Explosives skill of 60 and 5 C-4 plastic explosives).
 * WARNING: This will fail the quest, and the player character will not earn any XP. The player character will be transported to the outside of Vault 19 and the "quest failed" message will pop up.
 * This will also leave the ending for Vault 19 unchanged: despite being utterly destroyed, the Powder Gangers there will still continue to raid the countryside.

Quest stages
For Philip Lem:

For Samuel Cooke:

Behind the scenes
The name of the quest is a reference to the 1975 song Why Can't We Be Friends? by War.

Bugs

 * If following the Lem quest line, the map marker will tell you to go to the "Sulfur Caves" entrance (now blown up and billowing smoke) to find and talk to Lem about joining the Great Khans after talking to Papa Khan. Lem is actually located in the Overseer's Office, and if you've opened the stairs leading to the sulfur caves he can sometimes be found stuck there.
 * If you kill Motor Runner it will overwrite this whole quest and end it for an unknown reason.
 * If you enter the Overseer's Office from the elevator entrance (Hard Lock) in Sulfur Cave and open the Overseer's Office with the Average terminal, or just go through this route at any time, you may have some issues with your companions as mentioned above in less detail. When you ascend the stairs into the office your companions become confused and instead of following you up will run out the door back into the caves. If you are at the top of the stairs with a companion and run down they will run back into the vault. It is possible, however to keep your companions with you during this if you manually "push" your companion up and down the stairs. Just crouch and slowly move them and they will glide up, or down the stairs from your push.
 * This seems more like a path finding issue. Your companions will go back the way you came and reappear by your side a few minutes later, the same way they will if you jump from a ledge.
 * If the player chooses to destroy the sulfur caves and the vault, but didn't kill Cooke personally, then the ending sequence will still show Cooke still alive and the Powder Gangers terrorizing the Mojave.
 * After speaking to Papa Khan for Cooke and returning with the news, it is possible to enter Lem's office through the main door (not the overseer tunnel) to see the piles of bloody entrails. Returning to Cooke will not trigger the dialogue where Cooke admits to killing Lem; he asks you once again to go and speak to Papa Khan on their behalf even though this is now impossible.
 * This can sometimes lead to problems gaining full rewards for the quest, as the final option of informing Cooke will be marked as incomplete even though the mission will be marked as completed.
 * The dialogue options can be gotten by viewing the entrails first before telling Cooke about Papa Khan's decision. If you talk to him again after finding out he killed Lem, he will still ask you to go to Papa Khan.
 * If you have initiated the quest Oh My Papa and haven't triggered the conversation with Regis, the option to ask Papa Khan about Philip's/Samuel's group is not a dialogue option, making it impossible to finish the quest.
 * A detonator is not required to detonate the C-4 plastic explosive, although it is required to use C-4 during regular combat.
 * If you close the Vault 19 door then climb the ladder into The Wasteland before the door closes, the next time you try to open the door it won't work. You will have to climb back out and re-enter.
 * Sometimes when entering Lem's room via the passage under the Overseer's desk you will not be able to enter even though you have disengaged the lock from the terminal on the wall. An invisible barrier will keep you from entering. If you go back to Lem's room and walk down the stairs you can then leave the same way.