I could be wrong, but wasn't TOT the last time they demoed everything before going to record the album?
I believe you're right.
Although you almost can't even call that demoing. They wrote the songs in a practice space, had some live run throughs and then recorded them. The "demos" released on Ytse Jam were just a live version of the songs with no vocals. I'm unclear if they did in fact write vocals melodies at the time and just never put them down or if they waited to write that portion until after the songs were written. I think it's the latter although James was in the practice space with them.
But remember that with TOT, they had an initial writing session (and they may have done SOME recording--I don't remember for sure), took a break (the Escape from the Studio Tour, with Queensryche and Fates Warning), and then went back into the studio to finish up.
So are you saying they wrote more once they left the practice space and entered the studio? Because my understanding was they wrote the album minus Vacant in the rehearsal space, did the live run through (that is available on Ytsejam) and at that point entered the studio, did the tour, and then came back to the studio.
So I thought the whole album minus Vacant was written and live demoed at the rehearsal space and then they entered the studio.
The whole album was written instrumentally in the rehearsal space, and recordings were made of all the songs. Then they went into the studio and recorded (I believe) the bulk of the music instrumentally. From there, they went on the Escape From the Studio Tour and wrote the lyrics (and I'm pretty sure the vocal melodies, since the vocal melodies were the responsibility of the one writing the lyrics). After the tour ended, they returned to the studio to finish up recording the instruments and all the vocals.
edit: And to be clear, I don't know where you got the idea that Vacant was written after the fact. That may have been the case with when WfS was written in relation to LtL, but not here. According to the liner notes for the ToT demos, Vacant was song #5, written before TDS and SoC.