Yes. I've just tried Quicktime and it won't load the files.

What does that mean?

These are apparently not traditional MP4s. Assuming you're trying to add them to your project using Import Files on the Project Media panel.

What model of camcorder are they coming from?

You'll need to open these files in MediaInfo (a free download) and post to this forum the resolution, frame rate and audio and video codecs it lists.

They are not from a camcorder but from security cameras, so that might explain the difference. But I thought that if the common MS programs can read them, a powerful program like Vegas would have no trouble.
I will check out MediaInfo and post the results here.

Here's what MediaInfo says:

General
CompleteName : 19575075.mp4
Format : MPEG-PS
FileSize/String : 26.7 MiB
Duration/String : 56s 381ms
OverallBitRate/String : 3 972 Kbps

Video
ID/String : 224 (0xE0)
Format : AVC
Format/Info : Advanced Video Codec
Format_Profile : Baseline@L4.2
Format_Settings_CABAC/String : No
Format_Settings_RefFrames/String : 2 frames
Duration/String : 56s 381ms
BitRate/String : 3 893 Kbps
Width/String : 1 920 pixels
Height/String : 1 080 pixels
DisplayAspectRatio/String : 16:9
FrameRate/String : 25.000 fps
ColorSpace : YUV
ChromaSubsampling : 4:2:0
BitDepth/String : 8 bits
ScanType/String : Progressive
Bits-(Pixel*Frame) : 0.075
StreamSize/String : 26.2 MiB (98%)

Text
ID/String : 189 (0xBD)
Format : RLE
Format/Info : Run-length encoding
Duration/String : 55s 400ms

Could someone advise me on this please? I've posted the details as requested.
I have been able to do some basic editing with MS Movie Maker, and it seems silly that Vegas can't even open the file.
Many thanks.

I'm not sure how successful you'll be editing video from a security camera.

But, based on what you've posted, there is no reason you can not import a file with these specs into your project.

You are importing, right? Not trying to open it with Movie Studio.

Do other videos from this device import into a Movie Studio project?

Have you tried loading the file into Windows Moviemaker Live?

It may be worth $29 to buy Quicktime Pro from Apple. It includes an editor that will very very likely edit this type of video.

Yes I've tried both importing and opening. There's a number of these that I need to extract sections from. I'll have to try Quicktime Pro. Thanks.

Edit to say: Yes as noted above, MS Movie Maker does open these files without any problem - that's why it's so frustrating that a paid program like Vegas won't do so. But the MS program doesn't have the editing I need.

I think it's the text that is keeping the file from opening in Vegas.

Now there's a thought!
I don't know what the function of that text is. The files are from security cameras and have a time stamp on the screen, but I thought it was actually superimposed on the picture. Something to check out.
Thanks!