A good way to look at it is what does a charecter gain from building tanky and how does building tanky contribute to the team.
There are So Charecters who deal damage mainly the CD"s that have Good base damage and bad scaling, IE Morde, Garen He gains more from allowing to throw out a spell 2-3 times in a fight more then throwing it out once then having to back because of no health.
There Are what are currently labled Tanks, These have hard CC (stuns Aoe Prefered), Abilities that Scale or grant defensive stats.
Mages- Good Burst, Low surviability some trade Utility for better scaling in General built best with mainly offensive Items with 1-2 defensive onces as the game goes on
We look At Vlad
1) Does he gain more damage out put from building survability then he does from building damge? No
-Reasoning His pool allows him to Reposition himself in a fight if he gets focusd allowing for his spells to com up again with out having to build more defense
2) A true tank, no, A Charecter that looses % heatlh for each spell cast realy makes a badd Tank. Becuase the more EH you build the more the % health makes you loose, He ahlso has no strong CC and the only one he does have costs 20% of his current Health
3) Mage, Decent scaling, Good utiliy, Ult debuff and escape skill, And a passive that allows for more building of offesne making up for his not amazing scaling (anivia 2 1-1 ap ratios and AOE scaling Toggle ult,