Price to performance wise I was thinking the HD 7970 would be the best, but I guess the higher the price, the more performance your get.
I have no idea where you get that from.
Well you asked about the best card there, a 780 will outperform a 7970. But in terms of what's relevant right now, the 7970 would be the better buy as a 770 and 780 are overpriced.
In term of price for performance the HD7970 is the best since it will get the same performance as a GTX 680 while the GTX770 will get the performance of a GTX680 and a GTX780 will get the performances of a GTX titan.
Sony Vegas supports both OpenCL and CUDA, and with OpenCL being superior in performance to CUDA I'd go with an AMD card. The HD 7970 is also a mighty gaming card in its own right.