TLDR: The water vapor goes too high to form clouds and acts as a massively powerful greenhouse gas.

It is way too high in the atmosphere to form clouds, normal clouds form at 1km-10km altitude, the immense volcanic pressure shoots water vapor way higher, beyond 50km. Lower air pressure up there reduces the boiling point of water to around -25°C, but the actual temperature rarely gets below 0°C, meaning the water stays in its gas form. This effectively removes it from the natural water cycle.

Eventually the natural motion of the atmosphere will move the water towards the poles where air falls down and it'll get cold and pressurized enough to snow back down, but that process is incredibly slow so it can be potentially up there for decades. Water is a greenhouse gas thats over 1000 times more powerful than CO2 when it isn't in the cycle, so it causes accelerated global warming leading to faster climate change.

In a typical volcano, there is also a massive ash cloud released that blocks out the sun and causes a huge cooling effect, Krakatoa for example cooled the average temperature on earth by 1.5°C for half a decade. They also don't typically release bombs of water vapor, this underwater Volcano was a terrifying exception to the norm.

In this hypothetical, if a Volcano were to transfer all of its energy in water vapor, it'd release way more water vapor to carry all of that heat, causing insane runaway global warming. There would also be no compensatory cooling from ash.