Steve was well aware of how Veronica seemed to invite trouble and he wasn't taking any chances with her. While he still would have preferred that she was still back at his house, or even in the interrogation room until the vampire situation was under control, he knew she was stubborn and wouldn't take no for an answer. So she was here with him but on the condition that she would sit tight in his truck. Luckily, where Keith said he was wasn't that far from where he parked the truck so if Veronica was in trouble, he wouldn't be too far away to help.
Steve took a look around, seeing nothing moving in the shadows. He gave a small nod. "I will. You just sit tight." Steve closed the door, locking the vehicle. Steve kept vigilant and moved across the street and around the corner. Moving quietly and quickly down the street, he heard the sounds of a scuffle and he saw Keith trying to fight a vampire off.
Steve and the rest of the Five-0 were not in any way trained in dealing with vampires nor did they know their weaknesses. As of recently, vampires only existed in horror movies and stories. But he had to get the vampire off of Keith so he raised his gun, aimed and fired a few rounds at the vampire. After being hit, the vampire dropped Keith and stumbled away a few steps, looking dazed and Steve took this opportunity to run over to Keith. "Sir? I'm Steve McGarrett. You alright?" He quickly gazed over Keith for any serious injuries. He didn't wait for an answer as he started guiding Keith in the direction of his truck. "We have to move."
Before they could get far, the vampire seemed to have recovered from the shots and made a move towards Steve and Keith. Steve turned and put himself between Keith and the vampire. "Go! My truck is around the corner! Veronica will let you in! Go!" It probably wasn't the best first impression that Steve had handcuffed Veronica to the steering wheel but he'd explain later.
For now, he needed to keep the vampire at bay to give Keith enough time to get to the safety of the truck.