That really depends on the type of vegetation, the game type and the engine being used.
Is it for "Amazon Ant FPS Simulator 2.0" or "Orbital Strike Commander, Desert Planet Edition"?
How close you get determains what kinds of detail you need, What kind of models get created depends on the game too, how the engine handles transitions between each LOD determines how you make your assets.
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There is a massive body of information between the PC wiki and Google. I'd recon a guess you're answer is somewhere out there.
If you're having specific problems you will need to post images or an explanation in order for other people to help remedy your issues.
Is it for "Amazon Ant FPS Simulator 2.0" or "Orbital Strike Commander, Desert Planet Edition"?
How close you get determains what kinds of detail you need, What kind of models get created depends on the game too, how the engine handles transitions between each LOD determines how you make your assets.
Lots of unknowns...