In first method you have to install QuickTime from Apple website (Link can be found at the bottom of post in downloads section). After installation of Quick Time in your computer, you have to restart your computer. Now when you will open mp4 files in Sony Vegas these files will open without giving any sort of error. Many people have also reported that they got this error fixed by downloading and installing k-lite codec in their computer. If installing Quick Time didn't helped, give a try to k-lite codec.
If first method didn't helped try this one. In this method you have to convert all your videos to any other format that is supported by Sony Vegas. This method is little bit frustrating but keep in mind some thing is better than nothing :) So to convert your files to other format you need to have video converter, that will allow you to convert your video files to other formats. You can use any sort of video converter to convert your videos . I use Avidmux , you can also download this free video converter from the downloads section at the bottom of post.
Once you have that video converter convert your files to MPEG4-avc format or any other format that is supported by Sony Vegas and now when you will open that video in Sony Vegas you will not get this error.
K-Lite Codec Pack
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