Thanks guys for all your input! I found and easy solution on Vegas forum, if anyone interested – XMedia Recode is a great tool to convert m2ts to mp4 without losing any quality, and that’s the file Vegas Pro 14 accepts with no hustle.
Thank you! I was suspecting this… It’s just really strange that the TV station I need to send my file to asks specifically for IMX format (basically, they want an IMX30 625/50 (30Mb/s) compressor) and allows only 2 channel audio? Could it be that I choose the wrong template to render my file?