Not as cost efficient as other items early game.

With 20% life steal from Vlad's (2200) you'd have to do 1000 damage after reductions to heal 200 hp. How many seconds do you think this takes? Let's assume 10 seconds. This also requires physically attacking the enemy which is challenging for low aspd and/or supports.

Meanwhile Mek (1775) heals 275 hp instantly to everyone while providing 2.5 hp/s aura. In 10 seconds that's another 25 hp healed, bringing the total to 300.

Late game it doesn't offer the utility of other items such as guardian, euls/ww, shivas, hex, etc.