I must've performed thousands of Vorpal but still, normal TP return on a fully connected one appears to be 14%, Warrior's Charge usually produces a 15% one while all my damage records (
including 1600+ ones on Skoffins, Decos and Sea Puks) were 16%'s, no exceptions.
The only Store TP items that made it into my WS gearset are Rajas Ring and Brutal, therefore (and thanks Kwon for clearing up a couple things) my base TP should be 5.3 pre-Store TP addition, in WS configuration (.85 from Dual Wield since Suppa gets removed while weapon skill'ing).
(5.3+5.3+1+1+1)*(1.06) = 14.4 TP
...on a 5 hit Vorpal, thus, since I've never noticed a TP return greater than 16% (and aside from paying maniacal attention to my character's performances, as people must've noticed by now I sorta get off on WS big numbers) I think we can pretty safely assume that DA can only get triggered twice during a Vorpal Blade as anything/NIN.