I think Vic and Cassidy both have fairly reasonable story bits that justify their following the chosen one for some period of time.
Hey, you said some time, I did mention some time. Not to the endgame in some dangerous venture on oil rig miles away from the coast! Which at least 1 and a half years. (or with thorough walkthrough 2-3 yrs)
For Cassidy his bar is constantly being raided and he's treated as a second class person in Vault City, so getting the heck out of the place is a reasonable solution. An older wasteland survivor would know you can't make it alone and he was sort of stuck in VC without someone to back him up in moving on.
Yeah, no. Cassidy settled down in Vault City not because it's a nice city but because he needs medical treatment for his heart. As far as the game tells, he didn't even find heart pills during Chosen's adventures.
In Vic's, he was delivered from slavery by the guy and a little bit of info that isn't all that helpful regarding v13 isn't much of a repayment on that debt.
Vic is not very helpful for Chosen One aside from being mule, even his sprite kinda tells it. Besides, it is always possible to left him with his daughter in VC, well they have a lot to discuss.
/EDIT
-Vic at least can travel some time, to Vault City for example but he has his business to attend to in Klamath after all. And his info while not being plain helpful, does give at least a right direction.
-Marcus is a mayor of mining town with hard times when he needed the most even after the quest chain is over. It's over only for Tribal One, not Marcus!
-Cassidy without medical treatment won't last long normally.
-Myron getting all he need believes in speech of some tribal who rather belongs to the stables and goes away into anal enslavement treated worse than previous time, losing everything he had. He also should understand later that he's actually in danger walking around with the crew like hostile super mutant and another tribal.
-Goris senses something bad about his vault and go full "screw you, I'll go home." Alright, his revenge can be understood but it's way late in the game, let's be completely honest.
-Maria/Davin. Ha-ha, marriage business in Nevada, we get it. Ha!
-Sulik. Where's his sister anyway? Shouldn't he politely abandon Chosen One after some time when he obviously does not searching his sister with this chosen jackass? (alright, fixed in Restoration Project, we can find his sister, yay, mods will fix it!)
-Skynet. Shouldn't his curiosity and autonomy also contribute to the decision of lefting Chosen Tribal alone, especially when core region is pretty much explored, I mean later in the game?
-Dogs. Dogs are dogs, no comments here BUT. One is OK, more is turning party into fucking nursery which is near stupid.
-Lenny. While he does not have to leave, others shouldn't seeing this one in the party, rather than just leaving tongue-in-cheek comments?
Exactly. And this is what I liked in the older titles, plus New Vegas, in contrast with F3-4 : people don't just follow you for no reason. They have a solid reason to do so.
New Vegas is a good example, you have to bargain with Cass, and explain to her why it's a good idea, after you have cut all her ties to her former life. Arcade even asks you "Er... why would I follow you anywhere ?"
I'm tellin' ya, check 1.5 Resurrection. This mod nails companions, though it's not many of them.
Creation Engine physics 
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