Lute spent hours wiping to it so we have a good record on his stream archives.
If it's timer based it's inexact.
If it's from pull it's VERY inexact.
My team times off the first TP move and it's been pretty consistent (no 'early' Vivis if/when it reaches that point in the fight).
https://www.youtube.com/watch?v=bvUiKF4bItg
Code
Timestamp (time remaining) Event
===========================================
12:25 (47:57) Aita engaged
12:31 (47:50) Aita readies Fulminous Smash
15:39 (44:43) Aita readies Vivisection
18:53 (41:29) Aita readies Vivisection
21:59 (38:22) Aita readies Vivisection
Party exits/re-enters
43:56 (16:26) Aita re-engaged
44:04 (16:18) Aita readies Eroding Flesh
47:10 (13:11) Aita readies Vivisection
50:17 (10:03) Aita readies Vivisection
53:25 (06:57) Aita readies Vivisection
56:32 (03:50) Aita readies Vivisection
59:38 (00:44) Aita readies Vivisection
IF someone else feels like watching his other VODs to see if it's another countdown (number of TP moves, if procing blue !! modulates it, etc) have at it. Timer methods seem good enough for most groups learning the fight.
Other source on 'from the first TP move'
https://x.com/long_horned/status/1590166921902972928
'Vivisection comes approximately 3 minutes and 5 seconds from the first special move, and then every 3 minutes and 5 seconds thereafter'