As insane as it sounds, I believe the Vex have guardians as reinforcements. Here's why:
After reading all four ghost fragments of Vex, I think a Vex [i]explorer[/i] studies it's own individual. [spoiler]there were three researchers(including Dr.Shim!) and possibly three Vex units[/spoiler] Once it is a [u]perfect[/u] copy of the studied Guardian, the explorer makes multiple copies of him/her. After that, that Vex sends each copy into a ready,[i]empty[/i] Vex.
Please tell me what you think. This took me a while to think about.
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Vex shares one memory. All of them are connected. Simple