There's a pretty wide range of video codecs between Prores and MP4. How about focusing on one format or codec at a time until you can figure out what's up?
What model of camcorder is your MP4 coming from? When you open the file in a program like Media Info or G Spot, what resolution, frame rate and video and audio codecs does it display?
What happens when you try to import this video into a project that is set up to Match Media?
I just tried a Quicktime iPod video in Vegas 10/64 on Windows 10/64. It works as expected. Vegas says it's a h.264 codec.
EDIT: I'm using the latest Quicktime Alternative.
I'm outputting MP4 from Davinci Resolve but really is not a codec that I work with. Still, I was surprised that Vegas is rejecting it. Just a few days ago I was working with these codecs in Vegas12 under win8.1 without issues. I have only upgraded to win10 since then and havent touched anything else.
The Prores that I'm working with is coming from a Blackmagic Pocket camera (LT and regular Prores). Mediainfo reports all the correct specs, 24p, Prores, 1920x1080, 81.8Mps, 4:2:2.
Also, Prores is working fine in Resolve which makes me think that the codec is properly installed and that the video files are not corrupted. It has to be something specific to Win10+QT+Vegas.
I will try qt alternative as suggested as well. Thank you guys.
I just upgraded my main system to W10 and I don't seem to be having any problems opening any quicktime files. None were generated by Resolve but I have several flavors all coming from either a mac or vegas and they all seem to be fine just as before I installed W10. I'm using Quicktime Pro 7.7.4 and Vegas 13.
Not consciously slicking to any version of Quicktime, just happens to be the last one I installed. I don't have auto updates turned on for any Apple products, at least on my main editing system.