You cannot import things from Vue into Maya.
The xStream plugin for Maya is also buggy, unstable, and generally not fit for use. I have tried xStream 6.x with Maya 2008, and more recently xStream 7.2 with Maya 2009, with the same results.
The best way I’ve found to integrate the two is simply use Vue as a sky generator and composite your scenes in post-production. Even the standalone Vue 7 Infinite is terribly buggy, unstable, and generally unfit for production. Small, simple scenes work fine, but for complex scenes… Nightmare.
Correction: You can import “things” into Maya, but only terrains and primitives for example. Not ecosystems. You could, for example, export a sky as a spherical texture and use that in Maya. I have also exported Vue terrains, sculpted them more in Mudbox, and used those in Maya, but this is rather redundant as opposed to just modeling a terrain in Maya/Mudbox directly.
So the main thing you cannot export would be the Ecosystems, which would be the most important part of Vue, obviously.