This article is a good discussion on how GPU acceleration works with Vegas. Particularly useful if anyone is about to upgrade a GPU.
To TL/DR it, AMD cards outperform Nvidia in Vegas.
However, Hitfilm 4 Pro's most recent update offers improved performance on Nvidia cards with 3D models and particles. @Ady this implies CUDA? Or did you guys tweak OpenGL implementation? Or is there something custom/proprietary going on?