Vladmir, is not really a tank, but he ends up acting as one. He has strong aoe control, and sustain. He can literally heal himself over and because of this, hes an excellent team fight oriented champion. Perhaps he doesn't have a stun, but with the combination of a rylze, spirit vasage, protobelt, and even zhonyas. He becomes an incredibly difficult champion to kill or mitigate away from your damage dealers. In this sense, he acts as a DPS mage, but at the same time provides aggressive peel for your primary damage dealers. Or your back line of the team.
In a simple form. He can face tank most of the enemies burst, heal it back up create marginal seconds of disaray in a team fight, removes the opponent back line or atleast weakness them enough to force summoners, or put in kill range. Vlad doesn't need a strong team comp to fit, and he can do all of this by himself. Someone new to vlad or who doesn't understand his mechanical advantage will fail at this( they don't itemize utility, and go for straight AP), but vlads who are decent or even excellent, will create this situation( and have itemized with utility, and will have a greater win chance).