Fixing spellvamp isn't about the champions you think 'abuse' it.

There's not a reason why Vlad is 'broken' due to his sustain when ADCs have access to even more.

Spell vamp will be bad on other champions because in the past, when values were larger, it was practically a must-buy stat due to the tanky meta in seasons 1 and 2. In addition, the majority of spells are either long cd or AoE, making spellvamp less effective on them. Unless that tanky meta with 8-15 second teamfights comes back, its very likely that spellvamp will never see consistent play again.

If you cap it at anything you might consider 'balanced' it will still be a nearly un-used stat. With how quickly teamfights are decided, its very unlikely any mage will get use out of spellvamp outside of the ones that its core on, especially with how squishy most mages are.

If you cap it, all you're doing is ruining the characters that can currently use it.

The AoE/DoT interactions must be addressed if its going to be viable on more champions. They are everything that holds spellvamp back without breaking it on the champions that currently use it.

As a final note, Riot themselves stated that they didn't want sustain to be a common part of the laning pattern mid. They will make it a more appealing option for more champions (likely no more than 5 or so), but its not going to become what lifesteal is for AD champions. Especially since ranged don't get any penalties for lifesteal compared to melee.