Hi Roberto, thank you for the compliment...:)
I am not an expert, but Sony Vegas has its own collection of codecs that are not necessarily accessible by other applications.
Usually, if you can play the video in Windows Media Player, you can use it in iClone. This has something to do with DirectShow and VFW (Video for Windows).
Now there is a specifc VFW codec that handles H.264 streams and that might work for you. It is recognized in iClone. Here is a blurp about it:
Komisar's unofficial x264 VFW Codec, based on x264 builds, allows you to encode and decode H.264 streams in any Video For Windows compatible program, such as VirtualDub.
x264 VFW Codec allows you to configure the full range of x264 encoding settings through a Windows interface (as opposed to x264 command line commands).
x264 VFW Codec is available in 32-bit and 64-bit builds.
Here is the link to the page: http://www.digital-digest.com/software/x264_VFW_Codec.html.
I don't have .MTS files so I cannot verify, but it might be worth looking into. Best to set a restore point in case you install, just as a precaution.