Spell vamp isn't at 33% efficiency on AoE to spite AoE and make single-target better, it's there because spell vamp would be fundamentally broken if it worked at 100% on AoE.
You can easily hit 3 targets with every AoE ability cast. In lane there's minions. In the jungle there's monsters, in team fights there's enemy champions. In skirmishes there's a combination of those.
Not to mention minions have essentially 0 MR, and you will do true damage to them with just M Pen runes. Dealing 300 damage to an enemy with 50 MR (33% DR) means you effectivly deal 200 damage to them and restore health based off of that 200 damage.
If you deal 100 damage in an AoE skill and strike the enemy champion and 2 minions - you deal a total of 267 damage and would heal off of that 267 damage.
Single target stuff isn't triple the damage of Area Effect stuff though: and AoE mage like 
does ~1000 damage with his full rotation provided you only hit 1 mine and take the 80% damage from the side of his ultimate, where a single-target assassin like 
does ~1300.
Base burst actually doesn't differ to much from champion to champion, single-target champions mostly just have lower cooldowns to allow them to feel re-set-y in team fights: though considering champions are often the only worthwhile targets, there's less incentive for mana-sustain items to spam those skills for spell vamp gains (also they tend to cost as much or more, which really kills your mana if you're not running blue buff).
Drawing out healing mostly helps AoE champions as well, as single-target is usually enemy champion burst, where AoE tends to fall more into the poke/wave-clear/siege zone of semi-sustained damage.
Also - most of the champions that consider spell vamp currently have a lot of area-effect stuff in their kit.
"Manaless" is the big draw for spell vamp when you look at who actually gets it. Which makes sense - if you're so reliant on abilities to do damage you go for spell vamp, running out of mana is seriously bad news - and spell vamp kills you mana sustain in lane to really offer meaningful returns.
, 
, 
, 
, 
, 
, 
, and 
are the champions I've seen with it - the last 2 aren't manaless but they do have a 0 mana cost way to benefit from it.
Considering the way and deal damage, drawing out the sustain on sustained damage anyways would likely see a sizable boost to their power - in the ~200% additional spell vamp zone, depending on exactly where you set the maximum healing per 5 cap at.
Also, regardless of what cap you have - it's a noteworthy buff to 
, seeing as she comes with a rather substantial amount of spell vamp through her passive and the bulk of her damage is a little drawn out.
For that cap - keep in mind the 5 health per hit on 
used to have was actually pretty strong in regards to the 3% life steal it has currently. You need to deal 167 damage to exceed it. And that's deal 167 damage, not have 167 AD. Realistic and fairly low amounts of armor can require 230+ AD.
Which raises serious questions about how effective a cap could realistically be.
Also - if the cap is a static, set amount throughout the game - you allow for a massive power spike early when spell vamp will never hit the cap, which REALLY helps manaless champions that can actually afford spell vamp in lane. Which is most of them.
It's an interesting idea, but you're just pushing area-effect way to hard with your current suggestions as every change proposed dramatically benefits them.
Anyone?
