Psychologists vary in precisely what they mean when using the term
hyper[-]vigilant to describe a personality type. What is common across notions and here relevant is an acute concern about — and sensitivity to — behavior by others that may carry information about intention, about propensity, or about capacity. Hyper-vigilance typically arises as an attempted adaptation in response to seriously hurtful experience; it is in any case a self-defense behavior more focussed on identifying hostile or otherwise threatening intentions, propensities, or capacities, and should be expected to be associated with other defensive behaviors and more generally with personality attributes that arise from injury.
Hyper-vigilance itself is not the same thing as paranoia. When there is an element of irrationality to hyper-vigilance as such, it is in an over-commitment of resources to the tasks of awareness or of interpretation. The hyper-vigilant may otherwise be for the most part rational in their interpretations of behavior. (And one cannot reasonably infer that there is an over-commitment of resources simply from the fact that a hyper-vigilant person is seeking greater awareness.) A paranoid systematically makes important inferences that are themselves unreasonable.
The skills of the hyper-vigilant (and, for that matter, the unreasonable inferential practices of the paranoid) aren't always employed for purposes of self-defense. People may be identified as well-intentioned or peculiarly talented, and cultivated as friends; people may be perceived as having concealed vulnerabilities, and quietly given protection.
When two people interact — whether either is hyper-vigilant or not and so long as they are at all social — they consciously or unconsciously each size-up the other. The behavior of each usually adjusts to anything learned in the present encounter, and that adjustment of behavior may then communicate something new to the other, causing a counter-adjustment on his or her part. When two people have complementary emotional responses each to the other, a feedback loop is creäted, and the responses amplify to some extent. These feedback loops can cause people to take relatively quick and markèd likings or dislikings each to the other.
When hyper-vigilant people interact, complementarity has a still more pronounced effect. They can move to attack or become friends or come to love or indeed fall in love with a speed that startles everyone — including the two people in the feedback loop if they've never considered the dynamic or if they haven't each discerned that the other is not merely ill- or good-willed but also hyper-vigilant. Because hyper-vigilance is a behavior of self-defense, it is likely to be accompanied by a suppression or masking of behaviors that would otherwise expose emotions or reveal defensive abilities or propensities, and hyper-vigilance itself would be one of those behaviors; additionally, a hyper-vigilant person may conceal vigilance to avoid censure (especially as hyper-vigilance is widely equated with paranoia). Thus one or both of two hyper-vigilant people may miss this important insight in the implicit challenge of reading the other, especially when vigilance is operating largely at an unconscious level.