I have seen this happen a few times. Basically the footage was captured with the automatic settings thinking the footage is something other than what it is. This normally happens if you press "capture tape" and it starts to capture from the very beginning, somehow vegas misinterprets the flags and your footage ends up looking like this. I have not found how to convert it back to normal except for recapturing your footage. Basically you want to make sure to pre roll your footage at least 5 seconds so that it does not start the capture process before your actually footage and settings start to roll.
i've also seen that happen when one attempts to capture thru USB using Vegas ... WMP plays clip normal ...playing back thru Vegas capture utility the clips play back normal ..but when you drop in TL = digital squares