don't know if you have solved your issue with this yet, but after a quick search I found this thread:

https://steamhost.cn/steamcommunity_com/app/294100/discussions/0/3960307437823694608/?l=english


and after looking in the Faction Defs of the Ancients mod, the New Vault tag is <li>VFEA_NewVault</li> -> which should go into the "your_factions_here" part of the xml file indicated by your patch instructions that you followed from

https://steamhost.cn/steamcommunity_com/app/294100/discussions/0/5188663423653160209/

i created a local copy of the ancients mod in order to access the faction defs.

I added the patch file (name) to the patches folder in the royalty folder.

I tested the ability to make allied/hostile in dev mode, and it gives me the allied/hostile message now, instead of ignoring the dev mode set faction relation action. hopefully this helps anyone else who has this issue.

so, uh, do i patch this in the ancient's mod's "FactionDefs" "Faction_Player.xml"?

eh, whatever, i'll just try it myself.

edit: i was wrong. you were meant to create new .xml file with the codes written above. you put the xml file in the mod's patches folder. TIL how to mod (but not really)