Another player unlocked a deed for controlling 4 systems in the same constellation while I was still ranked first place in progress for the deed of controlling 8 planets--I had 3 planets.

Do the Vodyani Riftborn have some unique mechanic that lets them control 4 systems without having 4 planets, or is this a bug?

the unfallen can use there vines to quickly grab systems, and other factions can do the same with having a large border growth by producing influence.

Lorco wrote:

the unfallen can use there vines to quickly grab systems, and other factions can do the same with having a large border growth by producing influence.

As mentioned above, the player symbol that appeared on the completed deed was for the Vodyani Riftborn, not the Unfallen.

I subsequently scouted out all four of their systems in this constellation (I know that they still only have 4 because the tooltip says I can take control of the constellation with 5), and even many turns later, none of their influence bubbles encloses any system beyond its own (they're blue):

What else do you have?

Foraven wrote:

Controlling a system don't always means having a colony there. For example if you have good enough standing with a minor faction, you effectively own their system. Horatios are really good at taking over minor factions...

Having good standing with a minor faction controls their system for purposes of this heroic deed?  Could you please explain the rules for that in detail?