Vestiges are Others that are artificially created, often seen as a middle-point between a simulacrum and a ghost and compared to shadows. In other words they are something that can be seen as either a flawed simulacrum, or a ghost that left a deep enough impression in reality that you can use that impression as a mold capable of memories and complex thoughts.
Though this makes them flexible and capable of being molded and altered, they are impermanent. Given time and external pressures they started to degrade and the degradation grows worse over time, to the point that it takes more and more effort and energy to keep them intact. Eventually they break down, but they can be shored up with other spirits and Others to become more Wraith-like.
The book Vestiges: Glimmers and Gasps contains a wealth of information on the subject.
Rose Thorburn created a vestige for her grandson Blake to help him fall into the roll of a practitioner and possibly to secure a heir. Because of this they count as one person as she now serves as his reflection and her voice carries the power to influence Others, as their family line are typically female. Because of their connection she draws power from him. Then it turns out she created Blake as the vestige rather than Rose to act as her protector.
Johannes uses them in his territory as playthings for Others, who wish to return to the days before the Seal of Solomon, at the admission of gaining power from them. He shores them up with mouse and dog spirits to affect the with his implement.