There is no way of doing that in CatOS. That has been done on purpose. The idea was that the null domain was a particular one where the switch was "gleaning" a domain name from the network. When you are in this condition, you cannot configure anything VTP related. Once you have a domain and an associated configuration, you can't go back to the NULL domain without releasing the VTP configuration. Now, the problem is that you cannot clear the VTP configuration without clearing the whole switch configuration. There has been an enhancement bug open for that (so that we could clear the VTP config without altering the rest of the config) but it was not committed into the code, probably because of lack of interest for the feature.
CatOS is behaving the way VTP was originally designed. IOS is behaving differently, and allows you to configure VTP in null domain and clear the domain name. One bad consequence of that (I think), is that you can run your network in the null domain and then suddently lose all your config if a bridge with a domain configured is inserted in the network. This because a bridge in NULL domain is in this gleaning mode. That's not very nice obviously... Note that I'm not very knowledgeable about VTP in IOS and this may have been somehow fixed.
Regards,
Francois
