From a quick look around their site it seemed as though they have their own unique compressor. You should take a look at one of your files with a codec analyzer to see what it needs to function correctly in Vegas.
get one here:
http://cleansofts.org/gspot.html
Jeff
2-Xeon X5680 @ 3.33 MSI Mobo 48GB DDR3 GTX 580 3072MB 16TB Attached Storage Win7 Vegas 11 x64