Some MP4's I download will appear in Timeline to be allowed to edit. Some it's just the audio portion and not the video. Others won't at all. Is there a component I need to dl that allows EVERY video form to be allowed to edit in Vegas?
No magic bullet is possible. MP4 is a container format that can hold a wide variety of video and audio codecs, some handled directly by Vegas, some handled by the Quicktime library, and some requiring external codecs to run.
Best thing to do when you encounter a file that opens incompletely or not at all in Vegas is to post the full file properties as reported by MediaInfo, your Vegas version, your Quicktime version, and let others guide you to possible solutions.