He looks to Papyrus, the same morose look still on his face.

"If someone who has a hard time being honest tries to open up to you, and you shout at them, what do you think they odds are of them trying a second time?"

He wasn't blaming himself for actions the vampires took, but for possibly creating even more distance between himself and Sans. Incidents like that could make a person distant not just from one person, but from others as well. Sans was distant enough already without the Psionic messing things up.