Remember when Luke fought a vision of Darth Vader on Dagobah, and he cut his head off fairly easily? That's basically what this was. Kylo wasn't fight Vader, he was fighting inside his own head. If that was really Vader, there's no way he would have won.
![ARGENTINA vs. BOLIVIA [6-0] | RESUMEN | ELIMINATORIAS SUDAMERICANAS | FECHA 10](http://i.ytimg.com/vi/C3wPtKNbAOA/mqdefault.jpg)
Remember when Luke fought a vision of Darth Vader on Dagobah, and he cut his head off fairly easily? That's basically what this was. Kylo wasn't fight Vader, he was fighting inside his own head.
If that was really Vader, there's no way he would have won.