They've shown Phaser proof barrels / containers in many phaser fights.
That'd depend on the setting. The barrel is not shattered or vaporized - but perhaps it is stunned?
In any case, the phaser vaporization effect does not propagate indefinitely in a given substance, and doesn't easily hop from one substance to another. In TOS, the victim of a phaser hit disappears, but the floor beneath him does not. Or, say, Valeris in ST6 fires at a kettle, removing the metal, but the organics inside remain. Quite possibly a weapon tuned to remove you from existence would fail to remove a random barrel from existence by miss or ricochet.
Would it be possible to utilize that property to create armor? Say, if you wore kitchen-grade steel, would the phaser remove that but leave your body unhurt?
Probably. But would that do any good? If your enemy knew the substance you wear as armor, he would just tune his phaser to remove both the substance and
your body from existence with a single shot. Phasers can handle "composite" targets to some degree: a TOS hit at a Klingon wearing a chainmailish shirt and holding a gun removes the Klingon, the shirt and the gun alike.
Perhaps you could surprise the enemy by wearing a different type of armor to every battle. But no single substance would in any way be phaserproof - only surprising combinations, preferably with a lot of airspace in between, would.