Still doesn't know how to get it to work.

Would be great if you can add an example
to the manual.

Can you explain what question you had about vec_rotateaxis? I'll then put the solution as an example in the manual.

I'm not sure if this function can achieve what I'm after.
Actually, I want to rotate Object A (Pic A) to face Object B (Pic B)
without flipping 180 degrees (Pic. C) when Object B moving overhead
of Object A.

You can use ang_add here, not vec_rotateaxis.

Ok thanks. Maybe I'm stupid, I've read the manual several
times but still don't understand what does the function do.

I'm just think that it must have some tricky things that
can't be done by existing functions, otherwise you won't
add it into the engine, right?

Yes, it does some tricky things, but I've implemented it on a user's request - I have no real life example from a game for it. An example where this function would be used were an object that spins with precession.

Alright I'm just asking for adding an example
about this function to the manual and that's why
I write my post here.

It'll be appreciated if you can do that in the
future releases.

Thanks.