Vargouilles are another of those things where I wonder how a DM is supposed to use them. They aren't fast, they aren't smart, they lack any defenses altogether, but at the same time they do have the ability to inflict long-lasting, if not permanent damage or even death upon unsuspecting players that's almost impossible to reverse at level 2.
The flying heads have bonuses to dexterity, constitution, and wisdom, and penalties to charisma and intelligence. They fly at a speed of 30 ft. (good), have the Evil subtype, and get Weapon Finesse as a bonus feat. Their one outsider HD can be exchanged for a class level.
Special abilities are a poison that causes damage from bites to be unhealable (largely useless against NPCs), an awesome AoE scream that causes paralysis and lasts for multiple rounds, and a very strange 'Kiss' ability that allows you to turn a creature into a vargouille over the course of a day. Given that it only works on paralyzed enemies, which don't have stellar lifespans anyways, I don't think it impacts LA in spite of being a save-or-die.
However, despite the advancement opportunities and shriek, vargouilles are probably still okay as a +0 LA. Having an nontypical body hurts, as does the lack of diversity in its abilities.