I personally use the K-Lite Codec Pack (Mega version, installed with the Lots of stuff option for all codecs). Though I currently use Vegas 12, I switched to it from vegas 9 some time ago and used the videos in vegas9 before without problems.
In OBS just make sure you use AAC as the audio codec under encoding settings. (MPEG4 is kinda always used for video, either using x264 or h264 (if you use quicksync or nvenc)and no matter if you save as flv or mp4 with OBS) Then activate the "Use CFR" checkbox in the advanced settings, and thats about it.
If that does not help, please tell us what exactly happens if you try to import such a video file into Vegas11. Does only the audio import and no video or do you get an error?