Simple:

* Volcano vents vent vaporous vapor.

* Fiery fumes funnel forth from the fiery fissures.

* Molten magma moves mightily through the mountain's mouth.

More descriptive:

* Volcanic vents, vast and volatile, vent with a violent vigor.

* The vent's vibrant, vermillion glow vibrates with a violent, volcanic power.

* Silent, slumbering vents suddenly spout scalding, steamy smoke.

You could also use alliteration to describe specific types of vents:

* Fumaroles fiercely fume, releasing fiery, fatal gases.

* Solfataras simmer and sizzle, spitting sulfurous steam.

* Lava flows freely from fissures, forging fiery formations.

Remember, alliteration is about the sound, not the meaning, so the most important thing is that the words sound good together!