I don't see the need for a distinction between "phasing" and "vaporization" here. The phaser phases things out of our realm and into some other; this could cover the witnessed cases of "vaporization" quite nicely. But let's speculate a bit more on how everything
we see could be facets of the one and same phasing effect.
Usually, the lowest-energy way to phase a thing out of the universe is to allow the effect to propagate swiftly from the point of impact to the nearest phase border - say, the border between the victim and the surrounding air, or the border between a lump of meat and the surrounding kettle. This is how the transporter works when phasing things, from the looks of it! It takes special effort to make it cross phase borders (say, to include seawater in the beam-out of whales).
Yet sometimes a partial effect is desirable, even if it takes more effort to "rein in" the propagation of the phasing effect. Slow, piecemeal phasing obviously hurts more, so there's a special sadist gun for it (the Varon-T of "The Most Toys"), plus the option of using a standard gun on the sadist mode. It's just the same effect, but applied more slowly, with various drawbacks and extra costs but with the highly desirable terror bonus.
But there is another reason for using an effect that doesn't propagate to the body-to-air border: if you can key in an effect that only propagates internally from, say, the muscle-to-body-cavity-fluid border, you have killed the victim with a smaller-scope effect, and saved your ammo.
How could that be? Well, in this model, stopping an effect's propagation at mid-arm or half-torso doesn't save ammo. The volume
of the target that gets removed is not proportional to the amount of ammo expended. We have seen that the propagation continues even after the attacker's phaser has stopped firing: the initial "injection" of the phasing effect is enough, and all propagation from there on is "free". But different settings may propagate through different substances, and perhaps injecting a phasing effect that removes body cavity fluids from this universe is less costly than injecting a phasing effect that removes muscle and bone. That is, once you remove the fluids, the victim assuredly dies, but the low-power effect doesn't jump into the muscle tissue and the corpse remains; but once you remove the muscles, the victim assuredly disappears, and because this was a higher-power effect, it also jumps into the body fluids within and removes them.
Still with me? I have failed, then.