Amd works better with vegas, and XT chips like 7970ghz, or 290x would be the best performers. Search and you will find countless threads on the subject. NAB is next month, you may want to wait to see how things shake out with gpu / vegas releases.
If you want something fast that works well, AMD R9 270, 280 or 290 with their respective X versions. These cards provide the best timeline performance but are not supported by MC AVC and Sony AVC codecs; those will be just CPU only renders. MPEG-2 renders are very fast with those cards too and NB T3 likes it too. If you prefer Nvidia, the last card that worked well is the GTX580; there are plenty on eBay.