As the Divinities know, somehow it’s Belief which generates the life-sustaining energy necessary to maintain one’s Identity within Reality and while lesser beings may suffice with simple belief in oneself, augmented by their close acquaintances, Greater Beings require far more “fuel” to justify and maintain their positions within Reality. If true, then Vestiges represent those few beings who’ve succumbed to Fate yet maintain a tenuous grasp in one final ultimate struggle to avoid leaving Reality completely, clinging valiantly to their position in History in the vain hope some lowly mortal will provide the brief opportunity to roam Reality once again in the only manner left them: bound to another’s mortal Soul via a willing Pact offered in exchange for the modest use of their once-great power.
If true, the presumably it should be possible, albeit quiet difficult if nigh impossible, for the Vestige to rekindle sufficient Belief in its existence to regain not only its lost physical form within Reality, but perchance even its once lofty position of power. Apparently the Lich Acererak has proven such an unfathomable act is possible.
As far as creating new Vestiges, it would seem a “simple” matter of securing your place within such that a sufficient number of individuals recall your name and deeds (good or bad) to subsequent generations, thereby fostering a continual Belief in your current existence sufficient to maintain your unique presence. Obviously those legends need to include some means of eventual reappearance lest you simply fade to an interesting, yet unimportant history footnote. IE: President Lincoln while a very important history figure in our Reality could not be considered a successful candidate for Vestige because there is no question whether he’ll return from Beyond, whereas King Arthur might.