VoT is a buff (+str and sta), and should be upgrade-able. You are given the spell at the App1 level; spells that can't be upgraded don't have levels assigned. (Unless, of course, VoT is not supposed to be upgrade-able and has the App1 tag showing erroneously; we all know Sony never makes that kind of mistake.) I have yet to find upgrades either, on merchants or on broker.
Strangely, if you look at the SK spell listings here on Alla or on other sites, VoT doesn't even show up. SKs get the spell at 19, so there appears to be a lot of confusion with it for some reason.