Thanks for the mod. I noticed that:
events\jagdtiger_overlord_vassalize_subjects.txt
line 564 is missing the equals sign in : if {

After I fixed that it seemed to work in the current game version.

Ah. Granted, having any federation type with vassals IS abusable, even non-hegemonies, since you can set them to vote as you do, no matter what the federation charter or their vassal contract otherwise says. Like, it's funny in my current trade federation, even the guy keeping asking to go to war with a neighbour then votes "NO" on his own request when I do, because he's a vassal with restricted voting :p

If it helps, the federation part is entirely optional. If you pick just Imperial Overlord as the origin, you'll have the vassals and no starting federation. (Though you can still take the traditions and make one later.)

I was stupid not including a semi colon, the origin ITSELF can be abused by simply removing the weaker vassal from service and then annexing them, with hegemon this is brutally overpowered in terms of vanilla (3 homeworlds worth of production without much trouble by year 2225 maximum), in this case if one spawns close enough it's still strong.

I think gestalt vassals are a bit much for a simple preference reason and a comparatively minor balance one, I don't see how a hive mind that's not a progenitor would make so there's more hives and possibly machines (Haven't tested what you did), and I simply like more non hives in the galaxy.

The mild balance concern is just that it allows for the above but even stronger since gestalts have significantly better early game production anyways, and even if you don't exploit that production bonus serves you better with subject taxes if you go that route (this part is very minor).

Hmm, not being british or spiffing, I guess my abusing knowledge is rather limited. Can you please tell me how the gestalt vassals can be abused?

Cool idea, Gestalts forcing gestalts is a bit much and this seems ripe for abuse like the hegemon origin is in basegame but thanks for the mod.