I have two three component vectors, lets call them $\vec x$ and $\vec y$.
I want to find $\vec z$ such that $\|\vec z\| = \|\vec x\|$ and the $x$ and $y$-orientation of $\vec z$ equals the x and y-orientation of $\vec y$.
EDIT: And the z-component contribution in $\vec x$ is matched in $\vec z$.
Basically I want to find the vector for an object that's falling just as fast who's heading is different. Is this possible?