You realize vamp scepter + dorans beats thornmail for around half the gold? Take ls runes and you'll heal more than you take.
Um... no it doesn't.
Thornmail deals it's damage based on pre-mitigation damage, meaning the damage you would take before calculating armor.
If the enemy AA is supposed to deal 1000 damage before calculating armor, they are dealt back 150 damage plus 25% bonus damage based on your armor, so at least 175 magic damage.
Life steal is based on the damage you deal, post mitigation.
If their AA was meant to hit for 1000, then with 100-170 armor (Depending on armor penetration) they will deal more likely around 500 damage.
and life steal quints gives you 15.5 life steal, that means you heal for around 77.5 health.
You still take 97.5 damage (A bit less given ADC still have at least 30 MR)