The vent usually evacuates the areas behind and below the fridge. Any water entering this space, would likely accumulate on the rear of the fridge and the surrounding wall and floor structures. A small amount of moisture may not immediately damage anything, but a prolonged exposure can lead to a multitude of problems.

I would immediately cover the vent with a plastic kitchen  pan, a wast basket, tarp, plastic wrap or anything I could find to shed the water. A day of rain could ruin any wood framing, interior walls or electrical components. Prolonged exposure can result in mold accumulation and structural problems. Definately try to cover the hole until a new vent cap can be installed.