Your processor code is using non-typesafe operations when it extracts the vertex position data into a memory stream, then pulls individual byte values back out of that stream. If you treat typed values as a non-typesafe stream of bytes, you then have to deal with all kinds of messy issues like processor endianess.
You could add code to flip the endianess where required, but an easier solution is just to take out all this byte stream stuff and use strongly typed operations instead. The Content Pipeline graphics object model is designed to give you access to everything you need in a fully typesafe manner. Check out how our Triangle Picking sample looks up the vertex positions, for one example.
XNA Framework Developer -
blog -
homepage