Procedure

Basically, we're going to use the dot product to derive a formula for getting the angle between two vectors

First, we need to find the forward vector of the torso, and the incoming vector of the brick (relatively trivial and implementation based, so left as an exercise to the reader).

Flattening

Cut out the vertical components of the vectors:

function flatten(vec3)
    return Vector3.new(vec3.x, 0, vec3.z)
end

Getting the angle

The dot product between two vectors is defined as |u| |v| cos x = u . v, where |u| means the magnitude (length) of the vector. We can rearrange that to get x as follows:

|u| |v| cos x = u . v
cos x = (u . v) / (|u| |v|)
x = arccos ( (u . v) / (|u| |v|) )

Magnitude

We could get magnitude as follows:

function magnitude(vec3)
    -- By Pythagoras' Theorem
    local x2 = math.pow(vec3.x, 2)
    local y2 = math.pow(vec3.y, 2)
    local z2 = math.pow(vec3.z, 2)
    return math.sqrt(x2, y2, z2)
end

But Lua has an in-built method for this in Vector3::magnitude

Calculating dot product

Calculate the dot product the vectors

function dot(a, b)
    return a.x * b.x + a.y * b.y + a.z * b.z
end

Combining

Then we can put it all together

function getAngle(a, b)
    return math.acos(dot(a, b) / (a.magnitude() * b.magnitude()))
end

Note that this answer will be in radians rather than degrees.