There is a built in way, it's called the IntermediateSerializer, it's linked in Shawn's post.
Using that will allow you to define Vector3's as just <SomeVector>1 2 3</SomeVector>, as well as most other XNA types. This can then be compiled in to a binary format using a content writer, which means it will load a hell of a lot faster in your game.
It will also allow you to set the [ContentSerializerIgnore] option on any properties which you do not wish to be contained in your .xml, and [ContentSerializer(Optional = true)] on any properties which you want to be serialized, but not necessarily required. This will allow you to then define default values in the constructor of your object.