[Yaroslav Dronov]"Would R7/R9 380/370 use GPU and help to render faster in Sony Vegas?"
The latest R9 series work well with Vegas Pro providing that your CPU is powerful enough to keep the GPU busy and PCIe bus has enough bandwidth to transfer the data quickly. It's all about having a balanced system. Just buying a better GPU doesn't always make things as fast as it could. Sometimes it just moves the bottleneck to the next slowest component in your system.
[Yaroslav Dronov]"Is there any problems with opencl like with cuda in 600+ nVidia?"
No problems at all. Vegas Pro is based on OpenCL so it prefers good OpenCL cards which is why AMD Radeon is better than proprietary NVIDIA cards for Vegas Pro.