The rules sayeth,
You can inflict damage to your target equal to your unarmed strike, a natural attack, or an attack made with armor spikes or a light or one-handed weapon. This damage can be either lethal or nonlethal.
You can't crit because you're not rolling to hit (see Critical combat maneuver?).
Sneak attack works as none of it applies to the attack roll per se ("The rogue's attack deals extra damage...")
You can use power attack, and the to-hit penalty applies to the grapple check ("When you attempt to perform a combat maneuver, make an attack roll and add your CMB in place of your normal attack bonus. Add any bonuses you currently have on attack rolls due to spells, feats, and other effects.")
The attack is not a separate attack action, it's part of the grapple check which is a standard action, so no vital strike etc. that change/apply to the actual attack action type.