To take full advantage of the GPU-acceleration in Vegas Pro 11, you will need a supported card with at least 512 MB of GPU memory.
GPU-accelerated AVC rendering
NVIDIA: GPU-accelerated AVC rendering requires a CUDA-enabled GPU and NVIDIA driver 270.xx or later with a GeForce GTX 4xx Series or newer GPU.
ATI: OpenCL GPU-accelerated rendering requires an OpenCL-enabled ATI GPU and AMD Radeon Catalyst driver 11.7 or later with an AMD Radeon HD 57xx or newer GPU. AMD FirePro GPU should have 8.85 or newer.
GPU-accelerated video processing
NVIDIA: GPU-accelerated video processing requires an OpenCL-enabled GPU and NVIDIA driver 270.xx or later with a GeForce GTX 4xx Series or newer GPU.
AMD: OpenCL GPU-accelerated video processing...