Not sure if Nightmare and Vortex fit so well. For one, Vortex have a top poisoner, but Nightmare have their own selection of poisoners. Artus is also problematic: Vortex is a clan with a Defeat bonus. That means you want to maximize your winners in your other half. I wouldn't play a DR that can't win like Artus with Vortex. Third, Vortex want to partner with a clan that can provide strong characters to anchor the 4th round: the Vortex bonus is useless on the last round. However, SOB is often negated on the last round as well (for instance: against Vortex!). SOB often doesn't give you a benefit on the last round, which means they pair poorly with Vortex.
Nightmare is not bad, thing is you have to choose the right cards to make a half deck. when using Nightmare, you will probably be trying to win 2 rounds, (due to cards like Artus and Cutey and Kenny Cr). that said, people often find Cutey and Kenny Cr kind of hard to use with their 6 power. then, Edwin and Pan is used.
Nightmare works very very well with Uppers, just saying. Nightmare GHEIST is annoying as hell too