I've figured out the best workaround for videos not working with Sony Vegas. It is very simple: you put all the videos in a folder, double click a .bat, and they become instantly remuxed into another folder and work with Sony Vegas with zero quality loss.
You want to get FFMPEG.exe and put it into a folder, in the same folder make a new folder called "remux", and then make a new .bat file and edit the .bat file and paste this in there:
for %%a in ("*.mp4") do ffmpeg -i "%%a" -c copy -map 0 "remux\%%~na.mp4"
This will remux everything in the main directory into vegas-compatible .mp4 in the remux directory.
You can also download the premade .bat+ffmpeg.exe+remux folder to save time here:
https://mega.nz/#!qMNAVC7L!0hjbS97V0pEe ... WTXvA1ts_M
Here is a video demonstrating the fix: