I don't understand how their EP is ocean if they primarily live on land. Wouldn't they prefer a terrestrial environment with more land? Or are they supposed to be sea-weed?
Eventually, their physiology and society had progressed to the point where they were able to wage a guerrilla war against their predators, using the planet's only landmass as their base of operations.
This isn't meant to imply that they live primarily on land, only that the landmass was ideal as a base of operations, due to the unwillingness of the Zhnosh to leave the ocean.
Long ago, the Laenfa were a peaceful civilization of aquatic vines with very few of their present adaptations.
This is meant to indicate that the Laenfa are primarily aquatic. They are able to exist without water for extended periods of time, but they still prefer to be underwater, since locomotion underwater is much more efficient.
I'm not sure i understand the how a vine relates to a leaf and both to an "individual".
A leaf and the small length of vine which it controls is considered an individual. These individuals can attach to one another forming longer vines, but these longer vines are not individuals. Looking over, I think it's clear enough in the description. Is that what you were asking?
I would find them more plausible if they used carnivorous feeding more frequently, or at least whenever they engage in much movement.
Being plant-like organisms, they probably find heterotrophy disgusting. Presumably only those engaged in tasks where they would often be without water or sunlight for long periods of time and engaged in highly active work, such as soldiers and spies would resort to it on a regular basis.
Also it doesn't seem plausible that a single leaf could support a human-like level of cognition, or that it would really have a reason to do so, since an individual (i think) can't really do much on it's own. To move around, it would have to coordinate with the rest of the attached leaf-consciousnesses. You don't really specify, but it seem right to me that a leaf individually has a simple mentality, but they can use telepath to link many leaf-minds and think deep thoughts.
That's pretty much the idea. A single Laenfa is inferior in terms of locomotion and cognitive abilities. This could probably be made more explicit in the description.
Also, they may be photosynthetic, but with all their other qualities, they really aren't plants. "Plant-like" yes, but so is a sea anemone or a sea squirt.
I'm not sure they adequately fit into any of the defined biological kingdoms. Calling them plants is probably accurate enough for most purposes, since the defining feature which most people most readily associate with plants is photosynthesis.