Is there anything in particular you are looking for from that thread? From your question it seems you have understood what you need to do, you render an mp4 from Vegas, either using the Sony templates or t he Mainconcept AVC/AAC templates. The resulting video will play fine in the latest version of the Flash player.
I guess it depends on which video player you are using, but I have found that there is no reason to rename the resulting .mp4 file to .flv in order to play it in Flash, but I guess there may be some flash video players out there that check the file extension. I use my own flash player.
> ...then renaming the resulting .Mp4 file with a .FLV extension.
I'm not sure where you got this idea from but FLV is a container with a very specific file structure and renaming something that is not an FLV will not make it an FLV and will probably confuse any player that is expecting the FLV file structure to be inside the file.
I would leave your extension as is and have the player decide if it can play it or not. Or even better, render as an FLV file if that' what you want. Renaming video file extensions is almost never a good idea (the exception being things like mpeg to mpg or vob to mpg, etc. because the files really are all mpeg containers)