Most of the video player apps on the app store will play rmvb files. I've tried just about all of them and the one with the most robust features available today is Rockplayer2. It's free with ads, but you can remove the ads with an in-app purchase. And it's iPhone 5 optimized.
If AVPlayer ever comes back (I suspect they left due to the ban on .ac3 codecs imposed by Dolby), then that would be my choice.