Just the other day I watched a neat video by the Feral Historian in which he argued the theory that the Mirror Universe is not actually a traditional "alternate timeline" that diverged at a particular point in history and just developed differently from there. He argues that it's a more amorphous psychoactive kind of place that is "recreated" whenever visitors enter it based on those visitors themselves. I rather liked it.
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I always thought he was supposed to be Felix, julians friend who wrote the holo program originally
That's a distinct possibility. Mirror!Bashir calls him Fontaine, but it could be an alias. It's also possible that Mirror!Fontaine is an android.
The alias theory is fun. I remember some old novel that took place in the Mirror Universe having a never-Borgified Annika Hansen working as a spy, and her spy designation was "Agent Seven" just because there's a rule of cool about these things.
I always thought that Felix modeled the hologram Vic off of a friend of his named Vic Fontaine. So, in the Mirror Universe, the Vic we see is the original friend.
Prime timeline Vic was never actually a hologram. Just a holosuite employee with a transporter and good reflexes.

Even in hologram form in the "normal" universe, he was unusually self-aware.
Remember the TNG episode with those 20th century frozen people? It's like that, but mirror universe style.
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