Thanks a lot for posting all this help, everyone. Been struggling to "replace" Vue as an art tool in my toolbox, and of course Maya is much more precise and accurate and easy to work with, especially in 64-bit. But I just couldn't get my plants to look good with mental ray, and I tried many things.
Been working on a personal project for far too long (over a year) and ditched it several times from Vue's instabilities. Last night after reading this thread I switched all my plants out to instances, and replaced my previous shader networks with mia_x networks. The difference is night and day!
Mia_x materials and instances on the left, Maya blinns/phongs and duplicates on the right. This is just a background section of my scene, and granted the sky on the right isn't helping the render, the plants all look much more vivid and lifelike. Using Onyx Trees and PFX. I'm not happy with the Onyx Trees and will swap them out in time, but the GOOD NEWS is, I can actually populate my scene with mid-poly and high-poly geometry now! And it's not dense enough for my tastes (the foliage of the trees I'm using), but the leaves are looking much better to my eyes now. Still needs lots of tweaking, but there is progress, and after a year fighting Vue to make this scene I'm excited to be getting anywhere.
Thanks again for the help and ideas.