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.

If Vedalken Shackles becomes untapped before its ability resolves, you won’t gain control of the creature.

If Vedalken Shackles leaves the battlefield, it’s no longer tapped, so the control-changing effect ends.