I think we're kind of confused on terms here. Spell Vamp doesn't get you any survivability. Spell Vamp is also only 33% effective on area effect spells (such as Gale). So if your Tornado hits an enemy champion for 300 damage, you will only heal 13 HP. That's not going to change the impact of a fight at all.

What Spell Vamp does get you is lane sustainability - effectively HP5 attached to your spells instead of in passive form. If you hit all 6 minions in a wave with a tornado for 300, multiply that 13 HP healed by 6. 78 HP healed, not bad - and you can do it every wave. You'll be able to lane longer and heal up any harassment done against you.

Items like Aegis are completely different. They mitigate incoming damage and allow you to survive burst damage from enemies.

+1 From me.
I like it. I played using it for 10 matches.
People pitched a bit of a fit because I did not want to take the Mid-Lane. But we had a Caitlyn. (I feel shes more suited to mid than a Janna is anyway)

My lane not only got first blood in most of the matches but we won very quickly. in 4 of the matches the other team surrendered. The build is un orthadox but it works out. I d oubted the ability of the spell vamp, but it really does make up for not getting a Shroud or an Aiges to a degree. =D

It is pretty good if your trying to be an ap carry... Janna needs more durability imo. I believe Janna should always carry a RoA.