Being transparent is a character "vice of excess" stemming from what would otherwise be a character virtue in the form of honesty. It is intellectually narcissistic, which is to say that it contains either the intellectual narcissism of ignorance or prejudice. One may be ignorant of how much privacy that another person wants to hear from you ("TMI" - too much information), overestimating their empathy, or making them uncomfortable, such as seen in the comedic series, Seinfeld, the episode called, "Close Talkers." Transparency may be prejudicial against those who value their privacy and want to be left alone. In this case, transparency can verge on the "obnoxious."
Someone who is "trasparent," is by definition, advertising their private selves to the point of losing the courtesy, diplomacy, and good boundaries of retaining mutual respect (and the enjoyment of slow discovery of the identity of the other person.) See anything on the internet for examples of the abuse of transparency. However, all human beings communicate on a spectrum between being overly transparent, and too chicanerous to gain the trust of their audience. This highlights the central psychological skill that counters vices: maturity, which contains the Observing Ego to scan the environment for its composition and needs, matching that to what we endeavor in that social environment.
As a vice which makes partnership within relationships fail to get to goals, and intellectually unattractive, consider this vice to be negative in many ways, including in sexual attraction, since it is "wearing one's heart on their sleeve."