I don't see why it's not part of the default UI. The first thing you get in Fallout is directions to vault 15, and the marker appears on your PC's map. I can understand it as an issue if the mission was to find some unknown location, but if the PC knows where it is, then I see nothing wrong with having the game indicate that for the player at their option. Sometimes a player might be returning to the game weeks after their last session.
The only [unforgivable] error with this, is to have the VANS UI point to a dynamic NPC on the move; but not in line of sight with PC. If the PC is supposed to find a person, and they know where that person lives, then VANS should point to the NPC's house; not the alley behind one of the town bars ~where he's passed out.
If anything VANS could [hopefully] be modded in as a default perk, and not force the player to waste one on a Pipboy function... or it could be modded in as a Pipboy upgrade module or even a firmware patch on a bought or found holo-tape. 