I was also wondering if the the "siphoning life" truly is both.
It's spell vamp.
You might as well consider life steal and spell vampirisim never applying on the same skill. There are exceptions, but you'll make things easier on yourself if you ignore them.
Also - life steal is specifically auto-attacks. Spell vampirisim is spells.
Neither stat cares what you scale damage with or what type of damage you do. They care about being an auto-attack or spell, and that's it.
Smite is an ability, and benefits from Spell Vamp. Riven's Borken Wings is an ability, it benefits from Spell Vamp. Darius's Noxious Guillotine is an ability, it benefits from spell Vamp. Warwick's Infinite Duress applies on-hit effects and is coded to operate as a string of auto-attacks for the purposes of on-hit items and when-hit armors, so it applies life steal.
I'd also like to note, barring 3 on-hit spells in the game (owned by
), on-hit effects as a rule don't apply other on-hit effects like life steal or spell vamp, so you're SOOL if you want to leverage spell vamp/life steal on most hybrid builds.
Irelia's Hiten Style damage applies neither life steal or spell vamp.
2)I run a mastery the that has arcane blade in it (Auto attacks also deal bonus magic damage equal to 5% of your ability power). So on every hit with Hiten style, depending on what you have for numbers on damage, you do 75 true damage at full (I think) and a certain percentage of AP and AD on auto. It seems to work well when I am facing a tanky champion not knowing what exactly to build to resist my damage.
I'd probably avoid that mastery on Irelia.
Generally speaking, AP and especially AD are more things you want to pursue in a late-game oriented build focusing on a lot of scalers to take your damage to the next level.
or

When you're a tanky DPS champion like Irelia designed to build some tankyness, AD and AP just don't have a bright future in scaling your damage by missing out on 1 of those awesome late-game scalers.
And AD and AP are very much balanced around those existing.
You're better off getting items with no reliance on late-game scaling for their damage.
A solid early boost in power for exchange of a lack of synergy late with offense. Or just something like Sated or Kingblade that automatically scales itself as the game goes on.
Technically defense scales offense anyways - as the longer you live the more damage you get to deal. As such, Tri Force and Wit's scale pretty well into late with tank items, where they'll tank a
to the point everyone considers them mid-game champions.
You're going to do more magic damage with
than you will pursuing AP for the spellblade mastery. Irelia's already nigh-impossible to counterbuild as-is, there's no advantage to gain trying to be even harder to counter-build, and the 30 AP from Sheen/Tri is pretty much all you're realistically going to have on her: an unimpressive 1.5 bonus magic damage on-hit.