Internally displaced persons are highly vulnerable. They meet the refugee definition in all sorts of ways, except they haven't crossed an international boundary. However, as I mentioned in my presentation, there's a big problem with accessing them. You can't access them without the consent of the sovereign government of the country they are residing in. That government may be actively engaged in persecuting them or be unable to control the territory and whatever non-state actors are persecuting them.
There are huge logistical problems that I think may be solvable on an ad hoc basis. To the extent that the source country stream has worked in the past, it is because in individual countries or in individual situations you can still find a way, but it is an institutional obstacle that you don't have when you're dealing with people outside their country of origin.