mmh if you really need to merge them down to the vertex level, I say your best bet is to convert everything to polygons and continue from there, but maybe you just need to put everything in a group and use it like that.
If you do convert it to polys, I use this method to get a perfect match
convet NUrbs to subds first
then subds to poly using level 0
and that works great
NOTE: Don't use NURBS, they are of the devil hehehehe, unless you have to