I have a 4.3 vortec, TH4L6OE, and NP233 out of a 94 s10 blazer
I want to put them in my rock crawler chassis.

On the NP233 what are the 3 vacuum lines on top of the t-case used for?

I’m using a solid front axle with manual locking hubs
If all they do is engage the front axle then I shouldn’t need the vacuum lines.
Or do vacuum lines help control the t-case?

They control front axle disconnect and 4wd indicator light. You will not need them.

Isn't one the vent hose?

Nope, the vent hose is at the very front of the case near where the adapter bolts. The vaccuum switch (with the 3 ports) is on the side near where the shifter goes in the case.