Jason was trying to stick close to Amelia, albeit behind Klaus' back because if he had known that he was pursuing another Hybrid, it would surely mean death for Jason.
His chase after previously said Hybrid led him straight into a chase after a Vampire who still seemed young, but quick on his feet. It hadn't been very long that Jason had been a Hybrid for, only about a month - he reckoned. But mostly stayed around Klaus, in case he had needed something done.
Jason could smell the blood, smell the humans that the Vampire had been chasing, and luckily for him, he was a tad faster then the Vamp. But before he stopped him from digging into the flesh, he would have rather waited and watched, maybe join in on the fun.