Interesting theory, but ultimately it makes no sense.

Siva has no connection to the Vex other than it being a swarm of nanomachines that machinoform everything if given the set directives.

And the Black Garden most likely existed far before the birth of humanity, considering the Sol Divisive were curated by Quria when it deduced that the Vex should worship the Darkness. If that’s not the case, then they are still extremely old. But the Black Garden is much older. Or ally least should be, but with the Vex you really don’t know.

For Siva to have entered the Black Garden, humanity would’ve had to had knowledge of it during the Golden Age. Which, as far as we’re aware, they didn’t. The gate to the Black Garden on Mars was presumably constructed after humanity abandoned it in the wake of the Collapse, after the Vex moved in and tried to terraform it into a machine world. Furthermore, how would the Garden just suddenly “become” Radiolaria?

Also, the Black Garden is not birthplace of thr Vex, but a place of interest. We don’t know where the Vex came from.

In all, It would make no sense for the Vex to be human technology.