I know it's a pretty broad question, but does anybody know if you can have "character slots" for Fallout: New Vegas? What I'm looking for is a system similar to how Dark Souls has different slots where you can make different "themed" characters. I am aware of different save editors, but I'm looking for something where I can choose different characters in game more easily.
No, that is not how the save system works and you cannot change with mods how the game saves.
What you can do though is to open the console with
^ (QWERZT) or
~ (QWERTY), depending on your layout and give the savegame a name, e.g:
save "SniperLOL" save "MeleePwnage"
That is as far as it can get with character management in FO:NV. For more information on Keyboard Commands, see this thread on Steam. You only need to do this once, e.g. after creating your character, if you overwrite it, it will retain the name. The name you give the save is also reflected on your harddisc.
ps.: If you care about achievements, you need to restart your game after saving with this method, otherwise you are not going to get any and they won't be rewarded retroactively.