The question in your first post is a bit contradictory, you ask "I wonder how the data is stored in files"? Then you move on to asking about whether you should use a dynamic vertex buffer, etc.
Storing animation/skeleton data in a disk file often does not translate directly into any particular rendering API (though sometimes it does for optimization, like on the Wii).
Anyway, it's still hard to answer your question because a "sample" type of answer will not be very scalable to your particular needs. Additionally, cutting and pasting some highly engine specific and application specific code snippets also doesn't do much good.
What have you tried? How well do you understand the concepts of skinned character animation and skeletal hierarchies? Start with Frank Luna's excellent PDF to get the basics down and then you should be able to pick the API du jour to accomplish what you want.
Don't let the D3D9/D3DX throw you off. Learn the concepts then you can translate them to D3D11 and XNAMATH very easily.