Hi again,
what is the matter of ignore or not third party codecs in Vegas?

I would strongly recommend you leave it on the default. With the Vegas default, Vegas will always with the Sony Vegas DV codec.

With the setting off, Vegas may use 3rd party codecs like Microsoft's, or whatever else you have installed. I highly recommend against this, since you can get into a confusion about studio RGB versus computer RGB colorspace.

You do NOT wanna get into CODEC Wars?!?? Oh no! It is when they start fighting for supremacy, that excrement starts hitting the fan.