Vertex tweening for animation is an outdated technique, and there aren't many good reasons to use it in modern graphics code. I'm fairly certain it isn't even supported at the API level in D3D10+ (although you could always still implement it yourself).
One of the big problems with it is that it requires complete copies of the vertex data for each keyframe, which is very expensive. Furthermore, since the interpolations between keyframe vertices are usually simple linear interpolations, you need quite a few keyframes to have a high-quality animation, especially if lots of rotations are involved.
The current standard technique for animation of 3D bodies as of this writing is skeletal animation. There's a lot of information on this available via Google and even on this site; at the heart of it, you define a simple 3D rig (or "skeleton"), and you animate that by storing a series of transformations to the bones that skeleton. The actual mesh data includes weight information per-vertex that indicates which bones affect which vertex at what strength, and that information can be combined on the GPU or CPU to animate the mesh.