So I am facing this problem that I KNOW I have faced before...the solution can probably be found somewhere in a couple of my old projects.

I have a unit vector that describes a direction.
I have an object that needs to be rotated accordingly. When no rotation is applied, the object points straight down (0, -1, 0).

So, as an example, if this direction vector is (0, 1, 0), the object could be rotated 180 degrees on the X or Z axis in order to point in the diretion of the vector.

I'm looking for the best way retrieve the radians to rotate the object to point in the direction of said unit vector.
If there are plenty ways to skin this cat, I'd really like the most "efficient" way to calculate this.