They use heat sensors on the nose to help them locate a vein close to the skin of the victim who usually are asleep, and therefore doesn't notice the bat as it gets close to them. The bat will usually land somewhere nearby its victim, then slowly waddle up alongside, before using its very sharp teeth to make a small cut into the victim's skin. The bat then squats on the ground, and laps up the blood as it slowly trickles from the cut. Within 15 to 20 minutes the bat is finished with the 'meal' and the bat can drink more than its own body weight in blood sometimes drinking so much it is difficult for them to fly home afterwards. It needs to consume large qualities because blood is mostly water, low in real nutrients.
Most victims never notice that the bat is there.
They just continue to sleep peacefully while the bat feeds. When people get attacked, the only evidence that a vampire bat has visited is usually the discovery of a small cut and blood-soaked sheets the next morning. And if you've been very unlucky, the next sign may be a case of paralytic rabies which may develop three weeks later. Vampire bats will return night after night to feed on the same victim if it is a covenient one. And they have even been known to move their roosts closer to their victims so as to avoid a long flight home afterwards.