You could take the vector dot product between any two of the non-zero vectors, divide that by the product of the lengths of those two vectors and that's the sine of the angle between them.
Heres the problem:
The methane molecule CH4 is arranged with the four hydrogen atoms at the vertices of a regular tetrahedron and with the carbon atom at its center. The axes and scale are: each of the six edges has length square root of 2 with center at (.5,.5,.5). The vertices of the regular tetrahedron are (0,0,0), (1,1,0), (1,0,1) and (0,1,1)
Find the bond angle between the lines from the carbon atom to two of the hydrogen atoms.