We tend to see these tutorials not in English so here is a fabulous Spandex over Foam Tutorial from Ruffle Butt Cosplay.
While the tutorial is made with Spandex, you can definitely use it with 2-way or 4-way stretch vinyl.
Here are pictures of cosplay armor using this method.
Photo: About.com | Manga
Photo: Cosplay.com | Goldy
BTW, this is Goldy. He popularized this method and is one of my cosplay idols. He creates workshops for this in Japan and is super nice. I met him in person and really wished I was able to take a picture with him. I hope our paths cross again!