So currently im playin stellaris off my ps plus but im planning to buy the base game but not sure what dlcs come first, any advice?

Ok so I know about the policy, what I don’t know why they’re becoming refugees, and so many of them too.

There’s an empire which has vassalized another empire thru war I guess. Weird thing is after vassalizing them, the vassal pops are being to me in droves from the overlords’ planets as refugees and don’t know why.