OK, let's think about Viper's Strike + Villain's nightmare.
If the target was unwilling to take an OA, he wasn't going to walk or run, anyway, so Villain's Nightmare does nothing, but, Viper's Strike does keep him from shifting.
If the target was willing to take an OA to get away (he would have actually moved away from you, so Villain's menace would have possibly done something), he'll take it from Viper's Strike, and, again, Villain's Nightmare does nothing, while Viper's Strike at least inflicts that OA.
Of course, if you have an ally to use Viper's Strike on, you could use Pin the Foe, instead, that would finally lock the target down, he can't shift, and you can cancel his move with an immediate interrupt... except, he can move twice, while you can only cancel movement once.
A simple /grab/ can imobilize someone, forcing them to burn a move action to /attempt/ to get away. Villain's Nightmare could have just inflicted the Imoblized condition - it's not like other level 5 attack powers don't do that.
Villain's Nightmare is a weapon keyword, 3[W] attack vs REF, which is nothing to sneeze at, I guess. It's damage potential is probably comparable to Hammer & Anvil (warlord encounter attack 1), but it's effect is virtually meaningless.