League-of-legends – use spells/items using the champ icons on the left

To interrupt another player's ability (regardless of whether or not it is channeled or cast time -- it's the same ways for both) you can stun them, silence them, or kill them before the cast-time completes.

All channeled abilities can be interrupted by the channeler if you press "S" (Stop), click to move somewhere, or your champion gets stunned, silenced, knocked up, or otherwise interrupted. Knockback alone is not enough to interrupt a channeled cast, as shown here

You mustn't mix up life steal and spell vamp.

Life steal steals on physical damage (meaning auto-attack for vlad). In fights, you very rarely use your auto-attacks to deal big damage because you're a caster.

Spell vamp steals on magical damage (meaning all of his spells), this is want you want if your plan is to steal more life.

• Zhonya's Hourglass is great because vlad's ult has low range but must be cast in battle engage to hit the most enemies and for the 15% extra magical damage. The two second invulnerability will allow you to stay alive in the heart of the battle if you get targeted.
• I also very often use Hextech Revolver with Vlad, but you need some AP. It's no use having 50% Spell vamp if you don't deal any damage.

Vlad spells cost life, the good thing about more spell vamp is you can get your life back faster with transfusions minion lines and you can cast life costing spells (specially Tides of Blood that stacks) with less impact on your life.

EDIT : I went to have a quick look at all the different Vlad builds. All (or nearly all) of the descent builds have hextech revolver in them with its evolution to will of the ancients - give more AP, more spell vamp and an aura :-)