It's possible I'm missing something, but it sure feels like a bug.

 

To reproduce:

 

1. With two wall symbols lit, but before heading to see the vithrack, have two party members stand on the floor symbols.

=> Observed: they light up.

2. Head to the tunnel to meet the vithrack.

=> Expected: the symbols go out when the characters move off them.

=> Observed: they stay lit. (May be intentional, but bear with me.)

3. Murder vithrack, despoil corpse of soul vessel, loot adra spear from sarcophagus.

4. Return to room with floor symbols.

=> Observed: floor symbols are still lit.

5. March two party members to stand on the floor symbols again.

=> Observed: nothing happens.

6. Wield adra spear, keep soul vessel in inventory (attempted to equip it in the shield slot but no luck), march up and down the dais.

=> Expected: something dramatic happens.

=> Observed: nothing happens.

Yeah, nothing happened for me either, even when I was wielding the adra spear (I also tried having it in inventory but not equipped), had the soul vessel in inventory, and had the floor symbols lighting up properly; walked up and down stairs and attempted to interact with urn all to no effect.