First, in MED you need to find the name of the bone (parent bone), that's probably the neck, in your case. Try rotating the neck bone in MED just to verify.

Then, in your code, while playing vertex animation (or frame animation) achieve via ent_animate(), add your code to rotate the "neck" bone achieve via bone_rotate()

try to look in my thread, for reference:

http://www.opserver.de/ubb7/ubbthreads.php?ubb=showflat&Main=32805&Number=267788#Post267788

Hope that helps.