I don't think they have at all. A lot of the speakers at the DNC have mentioned that Kerry volunteered to go to VN. The contrast they are making is obvious. And IIRC, he only "volunteered" to go to VN after he found out he couldn't avoid it.
Agree. The Dems have been making subtle and not so subtle digs about GWB's National Guard service. I wouldn't be surprised to hear Kerry himself make some reference to it using his Vietnam service. The Dems see this as a winner and also a good offensive tactic to put GWB on the defensive. McAwful has been using it since January at least.