Let's say an enemy would hit an ally with an attack roll of 20, And a swordmage uses Dimensional Vortex to relocate him and make the target use it's attack on a different target. Will the initial attack roll of 20 carry over to the new target or does the enemy make a new roll for that attack?
From Arcane Power, p53:
It's certainly ambiguously phrased, but I think the key here is actually the trigger. Dimensional Vortex is triggered by the attack hitting an ally, so the attack roll is already established. Therefore, when you change the target of the attack with Dimensional Vortex, the enemy's attack roll doesn't change, because you're already past the point of rolling the attack and have moved on to determining the effects of the attack.
The target makes a new attack roll.
Making a melee attack involves making an attack roll unless otherwise noted. Compare to the wording of Guardian's Counter:
Also it is worth noting that the target will always have someone to attack after the teleport, itself.