First, when you activate it, you have to choose a target, and that target can’t have power greater than the number of Islands you control at that time.
Then your opponent has a chance to respond; if the creature’s power becomes larger than the number of Islands you control before Vedalken Shackles’s ability resolves, it’ll see that it has an illegal target and be removed from the stack.
After you’ve successfully gained control of the creature, though, your opponent can blow up all your Islands and pump the creature’s power sky-high, but it won’t change control back. The duration of the effect remains “as long as Vedalken Shackles remains tapped” and is not dependent on the power of the creature; in fact, changing it into a noncreature permanent (where it will have a nonexistent p/t) still won’t end the effect!
Today’s Rules Tip written by
Jen Wong, Level 1 judge from Irvine, CA