You may choose not to untap Vedalken Shackles during your untap step.
, : Gain control of target creature with power less than or equal to the number of Islands you control for as long as Vedalken Shackles remains tapped.
12/1/2004 The creature’s power and the number of Islands you control matter only when the ability is activated and when it resolves. After that, only the fact that Vedalken Shackles remains tapped matters. If the creature stops being a creature, you’ll keep control of it.
12/1/2004 If Vedalken Shackles becomes untapped before its ability resolves, you won’t gain control of the creature.
12/1/2004 If Vedalken Shackles leaves the battlefield, it’s no longer tapped, so the control-changing effect ends.