never cared, but at end of some npc conversations, what's with the "..."

I surmise that they forgot to record certain lines for voice actors and it was too late to call them back. I also think this is the reason why Sarah from Vault 21 has two voices.

i can't tell if you're serious, or just joking.

It's a writing device that conveys a sort of unspoken moment of irritation, dismissal, or grim response-by-non-response. Example:

Evacuation worker: "The boat will only hold 10 people."

Police Officer: "Load it and get going."

Evacuation worker: "But what about the other people showing up soon? There's no more transport left! They'll die!"

Police Officer: "..........."

Evacuation Officer: "All right. I'm on it."

http://en.wikipedia.org/wiki/Ellipsis#In_writing

Well you do have to admit that I was at least right about Sarah.

No Bark has two voices as well, and I don t remember him having two voice actors when the game came out.

A chupachabra must have jumped inside his brain tryin to steal his knowledge. "..."

no, i get that much, i was more wanting to know why in the game. does it mean i have something they want? or are they just unhappy with me?

In the game it was used in a 'yeah, yeah....move along' type context. At least that's what i recall. It's been a while since i encountered it.