Vedalken Mastermind is a brazen tool of il-repute, known to me ever since I saw his combo potential clearly outlined in my Tenth Edition fold-out instruction sheet. I can't find the sheet, but Ben Bleiweiss does it far better anyway in this nutty Building on a Budget article.
Back to my specific circumstances. My EDH already had Reality Acid as a way to deal with planeswalkers and enchantments. Adding Vedalken Mastermind just seemed like the right thing to do. Not only do I get the phenominal one-permanent-a-turn kill with the Acid, but there's a host of other possibilities.
Blue is the color of control magic effects like Take Possession. What happens when your "possession" suffers an untimely demise? You send the aura back into your hand with Vedalken Wizard.
Sure I can recur Trinket Mage to grab more artifacts, but if I had black in my deck I could even do it to creatures in my opponent's graveyard.
Vedalken Mastermind, unlike similar non-creature versions like Erratic Portal and Crystal Shard, also has the power to return itself. If played correctly, the Mastermind is invulnerable to sorcery-speed effects in all cases except when your blue mana is completely tapped out. Your opponents are forced to use instant speed effects, probably direct removal. Which I think is the most valuable commodity in EDH right now.
My deck is certainly not built around these two cards…that way lies dragons of the strategic variety. But if your deck has little combos sprinkled throughout, it is only a matter of time before the chocolate crashes into the peanut butter.