He walks to the settlement you assign him too. It can take time.

Also, he can be killed in that walk if you are in the area. I lost the Vault Tec Rep once and just put if on the level 4 merchant bug. I later found his body in the random encounter spot in the alley just west of Goodneighbor and Mass Fusion.

I've found it's better to just hang around GN for awhile before exitting. this gives the game a bit more time to get VaultTecRep out of the general area. When I leave I'll typically go south in an effort to not get VaultTecRep in my character's loading radius.

All level 4 merchants are bugged. Sometimes they don't show up, show up then disappear, show up and are stuck in dialogue, show up get assigned to their store and disappear. Sometimes, there's no problem at all.