Is the target for an ability part of the cost?
You can't cast a spell unless it has legal targets.

When a spell resolves, it will be countered if all of its legal targets are gone.

However, your Manic Vandal spell (the thing that you cast to create Manic Vandal the creature) doesn't actually target. Instead, it just summons a creature. Then, after he enters the battlefield, Manic Vandal's ability -- "When Manic Vandal enters the battlefield, destroy target artifact" -- triggers, and you put the effect "Destroy target artifact" on the stack. If there are no valid targets, nothing is destroyed. But you still get to keep the Manic Vandal creature.