DO a search on the topic - this was somewhat discussed in the thread about what version of the MC Mpeg encoder/codec was in Vegas. Currently you already have the MC HDV codec - it is the mpeg codec. You can both import and export HDV natively with both Vegas 4 and Vegas 5. You currently can't capture or PTT HDV however. My feeling on that issue is that it is just the means of NLE interface and currently Vegas does not interface with the capture portion of the codec.
Ah yes, but the new MainConcept MPEG Pro package for Premiere Pro allows for MPEG2 editing that is handled in the same way Vegas handles DV. You can edit away and any MPEG2 footage which has not been modified in some way will just be copied to the new MPEG2 file with no recompression. It works very well.