Calvin is the head of maintenance in Vault 81, though he doesn't always see eye-to-eye with Overseer McNamara. Because of the constant upkeep needed in the vault, Calvin has been looking for more spare tools to use in his repairs.
PCPlaystation 4Xbox One Calvin enters an inaccessible area called Vault 81 Holding Cell shortly after he is spoken to. The "entrance" is at the bathroom to the left of Holt, Alexis, and Erin Combes' room. [verified]
PC To speak to him again on PC, open the command console by pressing the ~ key, type 153855.moveto player, press enter, and finally press the ~ key again to close the command console. This teleports Calvin to the player character's location. At midnight he will return to the holding cell. Alternatively, type player.moveto 153855 press enter and close the console to teleport the player character to Calvin's current location.
PC To prevent Calvin or other NPC from returning to the holding cell; go into the wall at the top of the stairs facing the bathroom directly to the left of Alexis' and Holt's room (it's viewable on pipboy map as Vault 81 Holding Cell) using the tcl command, select the door to enter, then immediately turn clipping back on so it'll actually let you enter the room. Remove all NPC from the room by using the prid command and moveto player once back in the hallway. May need to move them a good distance away as the children in particular will immediately try and return to the holding cell room which they can access by entering the bathroom door, or use tai to toggle their AI briefly and they'll stay in place. Once room is cleared, go back to the holding cell's door, select it, then type lock 1. Move POV so the "Unlocked" message disappears, then look at door again, it should now say "Novice Lock". NPC will not be able to unlock this and will stay in the halls if they don't have an assigned room. Remember to deselect the door before attempting to use the tcl command again to restore collision, otherwise it won't work.
Playstation 4Xbox One You can gain access to the holding cell by glitching through the ceiling in the bathroom that is too the left of the Combes' room. To do this you must grab the trash can that is inside the bathroom (look at it and holding X) and taking it to the bathroom wall nearest the far left stall. While holding the trash can put your back to that wall and strafe as close to the stall as possible without actually entering it. Now, push place the trash can straight down as far as you can. You'll notice you kind of levitate a little. Keep doing this until you are able to clip through the ceiling and end up standing on top of the bathroom. Look to your right and you will see a stairway through the wall. There will be a clipped door frame where you should only be able to see the handles of the door. If you walk off that ledge you can press X to enter the holding cell as you fall into the doorway. If you don't time it right, then you will fall down to the reactor room where you will have to head back up to the bathroom and repeat the process. Upon successfully entering the holding cell, you will find Calvin and are able to sell him all the tools you are carrying for 15 caps each.
As of patch 1.4, Calvin keeps going to the holding cell at midnight and becomes inaccessible, but he returns to work at 6:00 AM, thus allowing the player to sell him tools as intended.