It appears that a phaser vaporizes organic matter on maximum setting. Remember that in this episode several lower settings barely phase Yuta but the maximum setting completely vaporizes her with no visible remains in the episode.
In TNG Episode "Vengeance Factor"
RIKER: Stop. (The first shot is a mild stun. Yuta staggers, then reaches for Chorgan again. The next shot is higher. Then Riker puts the phaser up to full power)
RIKER: Yuta, don't do this. (She moves towards Chorgan again, and Riker vaporizes her)
However In TNG episode "Aquiel" we see that even to end up with gooey remains it would take thirty to forty seconds at the maximum setting.
In TNG episode "Aquiel"
RIKER: Doctor Crusher tells me that in order to inflict the kind of molecular damage found in Rocha's remains, it would take a sustained phaser discharge of at least thirty to forty seconds. That doesn't sound like self-defence to me.
Somehow in the TNG episode "Conspiracy" a human that was infected with the alien parasite was only stunned by a phaser set at maximum.
In TNG episode "Conspiracy"
CRUSHER [OC]: You must set your phaser on kill. Stun has little effect.
So my question is when does a phaser kill organic matter and when does it vaporize it? You can add in the "goo" as well from the episode "Conspiracy" if you like. If you could give some information as to why it works differently in different circumstances that would be helpful.