Picture every emotion a human has as water. When you feel an emotion it uses or broadcasts out a cup of water, but when you feel strong emotions like fear, hate, or anger it actually broadcasts out a gallon of water letting everyone know to stay away from you.
We feed on pranic energy, so it would actually be easier to drink this "water" that you are broadcasting than it would be to drain some from your body (it is also safer for you and attracts less attention to us).
We are adapt at taking this emotional energy and with a little practice we can tell what "flavor" or type of emotion is being felt by you. Those of us who don't know a lot about shielding themselves actually feel your emotions as their own not realizing that these feelings belong to someone else and will eventually get a headache if they do not get away from the source of the emotions.
Since feeling emotions broadcasts out energy which we either don't want to loose or can't afford to loose we do not allow ourselves to feel these; however, the stronger emotions are too strong to hold back because they are more primal and are connected with survival. This is the reason we seem so aloof and cold.
Scanning is the same thing as empathy only instead of feeling your emotions we taste the "flavor" (normally to find those who are feeling strong emotions).
Each type of human has a different pattern to their energy fields (auras). When we taste or scan your field we can compare the pattern to see what type of human you are. The more proficient psi-vampires can actually tell which type of vampire someone is, either in person or over the net.
When we scan someone's aura we do not see the color but we are aware of it. We feel things but do not see them.
We can sense the presence of another vampire before we see them with our eyes.