For "time", you should probably read no "year" soon, as Digital Polyphony producer, Kaz Yamauchi, said in China of a Vita version, "If we were to make it, we'd like to make use of the ability to play anywhere and the ability to use 3G at any time." He was there promoting GT5 for Sony at China's Joy Exhibition.
Given the gestation times for GT products, that sounds like a 2015/16 time release, which is very far away. If anyone wants to develop a realistic driving game for the Vita, you have a couple of years to turn it around. We do have WRC3 to look forward to and Need for Speed Most Wanted on the way, but lack that genre defining title that the potential Vita owner might be looking for.