The vorseal starts you at 9, first proc 10 like indigla said
I personally tested the Abyssea Atma a few weeks ago and to my big surprise the first proc I got was always TH9, meaning that despite me TPing in TH9 (8 from gear/traits, 1 from Atma), the Atma doesn't allow to break the initial TH8 cap.
I think I even made a post about it in this thread.
Years ago I also tested the opposite, TH4 for non-THF jobs, and in that scenario the Atma was working perfectly, first TH proc message was a TH6 one, meaning that the Atma allowed non-THF jobs to go beyond the TH4 cap for non-THF.
No clue if that got fixed, haven't tested it in years.
Edit:
I never tested the Kupower, which only works in old zones. I'm talking exclusively about the TH+ Atma.
Edit2:
We know that If you TH with the same amount of TH as the TH currently applied on the target (up to TH8 at least, unknown for higher values) you have roughly a 6% chance to upgrade on each dagger swing that lands on the target and hits for more than zero damage.
This is pretty much confirmed.
We also know that this chance is higher for hits under the effects of TA/SA and for Feint. Also for Weaponskills (which won't show the TH proc in the chatlog though).
We also know that the higher above TH8 the TH currently is on the target, the lower the chances to proc a level up (so less than ~6%). Altough it's been discussed wether this is naturally meant to be like that, or a consequence that we do not TH in more than TH8 because we were led to believe it would have no effect (some early tests by Melphina seemed to hint that TPing in more than TH8 might produce slightly higher level up rates).
With all of this said the big questions remain: can TH level up proc on multiattack procs? Can it proc from either hands or just from the MH hits?
Would be near to impossible to test. I wouldn't be surprised with the TH chance working only on the first hit of the main hand for each attack turn though.
Granted this would mean that the level up rate is higher than the approximate value of ~6% we observed so far, I think?