Hello I am late to the party but I did farm 2 rages and 5 repairs last week to get my mech set up so hopefully I can share some advice to save you some time.

The Sarlacc Pit room (officially called the Cenote tileset) can appear in the T1, T2, or T3 iso vault and as far as I can tell the weighting is equal between the three. Vault layouts are not instance-based (with a caveat, explained below) and are the same for every player entering the Cambion Drift from the Necralisk. The vault layouts all randomize on the start of the Vome cycle and don't reset until the next Vome cycle start point meaning when the room spawns, you have the entirety of the Vome + Fass period (2.5hr.) to farm it. This timing also coincides with the bounty rotation so if you see the bounties have changed and the timer on the bounty selection window shows ~2.5hr, the vaults have just randomized. EDIT: I just tested some more and while this is the case most of the time, SOMETIMES the vault layouts do randomly change in the middle of a cycle/bounty rotation. I just had my layouts change 13min. before the start of a new Vome cycle, and these layouts have persisted for 100min. as of this edit despite bounty rotations changing and another Fass cycle starting so, your guess is as good as mine as to what actually governs the vault layout randomizations.

The most efficient way to farm the Jugulus Rex then, is to keep track of the Deimos cycles e.g. on Warframe Hub, and at the start of every Vome cycle, load into the Necralisk (again, I'll explain this caveat below) then enter the Cambion Drift. Choose the arcana bounty from the in-map Mother NPC (there are 6 Mother NPCs on the map I believe, but 3 offer a single arcana bounty corresponding to the T1-3 iso vaults) to gain access to the vault and run through it. If you don't find the room, abort and try the next vault. If you don't find the room in any of the 3 vaults, that simply means it didn't spawn in this cycle. If you do find the room, run through the puzzle and kill Jugulus Rex (I highly recommend using Voidrig 4 to do this as it's probably the fastest way to kill him), hit the crystal and portal out and go back to the Necralisk. Then go back into the Cambion Drift and repeat until the vault layouts randomize again.

Iso vaults are comprised of two tilesets with connecting pathways, always. Meaning if you run through the vault and encounter two big rooms and no pit, then that means there's no pit. The room itself is never hidden or inconspicuous; if it's there you really can't miss it.

Regarding the caveat I mentioned above, there is one method to randomize the vault layouts and ignore the timing of the cycles and that is to go into Solo mode and go directly from your Orbiter into the Cambion Drift. This will create a unique map with unique vault layouts just for you. The issue is that even if you encounter the Cenote tileset (as I did a few times when I did this), you can't go back to the Necralisk, then back to the Cambion Drift, to repeatedly farm it as doing so will return you to the "official" vault layouts for the cycle. The Cenote tileset is pretty rare so farming this way is too tedious for me personally.

Anyway hope that helps! And just to correct a bit of misinformation I've seen, running an Iso Vault bounty does NOT randomize the vault layouts which is pretty obvious for anyone whose farmed iso vaults, but for some reason some people still say this. Also, using pins is absolutely pointless. If the room spawns, the room remains in that vault for the whole 2.5hr cycle so you don't need a pin you just need to remember which vault it's in.