So, now that my players are foraying into Floor 3 of the Vaults, I've had a little feedback on the encounter design from them. My players have pointed out a frustrating amount of encounters with single high-level monsters, and I cannot disagree that there's a lot of them. (My players did take an unfortunate path through the dungeon while the Barbarian missed a session, so they had a rough time with a 3 caster party.) I've seen the sentiment echoed on /r/Pathfinder2e as well, so I decided to take a look at my list of encounters and do some fiddling.
I have left boss encounters untouched, as they should be difficult foes. There are some boss level enemies with names that I have adjusted, but I deemed them unimportant to the overall story or otherwise not foreshadowed enough. I replaced enemies with an eye towards keeping thematic similarity, as many of the enemies foreshadow things on other floors or provide a benchmark to show the players how much stronger they've gotten. I have also aimed for similar encounter difficulty and same XP totals.
Rooms are numbered based on the Special Edition.
Floor 1: No adjustments. The mitflits provide plenty of cannon fodder, in my opinion.
B15 Shadow (apply Weak template, add 1 Mummified Cat)
B26 Zebub (apply Weak template and add 1 Imp)
B31 Skeletal Giant (replace with 2 Skeletal Soldiers)
C13 Jarelle Kaldrian - could be adjusted, but the PC's aren't really meant to fight her
C19 Aller Rosk (apply Weak template and add 1 Ghoul)
C27 Lurker in Light (replace with 2 Gnagrifs)
C40 Gibbering Mouther (replace with 2 Squirming Swill)
Floor 4: There are a lot of bosses on this floor. The Voidglutton is certainly a TPK for a party lower than Level 5, but is optional. Warn your players before they encounter it.
D7 Poltergeist (Replace with 2 Charghars)
D11 Evangelist Velstrac (replace with Weak Ostarius Velstrac and 2 Augurs)
E22 Viscous Black Pudding (replace with 2 Luminous Oozes)
E25 Bone Gladiator (replace with Harpy Skeleton and 2 Skeletal Giants)
F9 Destrachan (replace with 2 Skrik Nettles) - I realize that Skrik Nettles are scarier without a ceiling, but the players will probably have fun walking on the roof. This encounter is really meant to show the players that scary things can be let out of the observation rooms (such as the Shangriol Behemoth below). The Shangriol Behemoth is enough of a big setpiece that I think it should be left as-is.
F10 Frightened Irnakurse (replace with 2 Driders)
F25 Jafaki & Drider - since there's a lot of Driders now, I replaced Jafaki's Drider guard with a Mulventok
G8 Gug (replace with 3 Sceaduinar)
G17 Witchfire (apply Weak template and add 1 Weak Will-o-Wisp)
I did not adjust Floors 8-10, mostly out of laziness, and partially because I think they're very well-designed. I will probably look at them again when my players get a bit higher level.
I'm loathe to set my encounters on Easy Mode,but will add monsters for groups larger than 4 players (or adjust XP down accordingly). I feel like the monsters that are written to be there ARE there- it's up to the players to handle them (or come back later).
I'm not above the free archetype option which provides wonderful utility to small groups without breaking game balance, and also letting players hire NPC mercenaries from town if need be. This can be an NPC of the group level or two that are 2 levels below (the math works out).
I didn't fully understand what you were after in my first reading and response post- you have kept the encounter difficulty the same yet dispersed the challenge amongst several monsters rather than a single creature. This is similar in philosophy to adding creatures to balance an encounter for additional players rather than applying the elite template to a single creature. Thank you for sharing this, and my apology for not seeing this sooner!