There seems to be a 50% chance that any bloodbug (variety) will survive a VATS blow that should have killed them, and despite having no health will successfully tank as many subsequent direct VATS hits as you can dish out until they (at a random moment) suddently realise they're actually dead and die on the spot all by themselves.
In the larger swarms especially I can count on at least one doing this. They're really weak, but taking lots of damage from pinpricks by an enemy you can neither escape nor kill is a little infuriating. This is the only enemy type I've seen do this, and it's happened regularly since 1.4 but never before then.