BackStab confirmation script?

So what this does is it checks to see if you backstabbed someone from....behind but its not working for some reason. What i did was i put the code into the Blow function of a sword script i really don't want to explain this code so you will just have to skim through it but the point is the message is not printing.

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

function blow(hit)
    if (hit.Parent == nil) then return end -- happens when bullet hits sword

    local humanoid = hit.Parent:findFirstChild("Humanoid")
    local vCharacter = Tool.Parent
    local vPlayer = game.Players:playerFromCharacter(vCharacter)
    local hum = vCharacter:findFirstChild("Humanoid") -- non-nil if tool held by a character

    local Torso=humanoid.Torso.Position
    local vTorso=vCharacter.Torso.Position
    local distance=Torso-vTorso
    local normDistance=distance/distance.magnitude
    if humanoid~=nil and humanoid ~= hum and hum ~= nil then
        -- final check, make sure sword is in-hand

        local right_arm = vCharacter:FindFirstChild("Right Arm")
        if (right_arm ~= nil) then
            local joint = right_arm:FindFirstChild("RightGrip")
            if (joint ~= nil and (joint.Part0 == sword or joint.Part1 == sword)) then
                     local Player = game.Players:GetPlayerFromCharacter(hit.Parent)
                                if Player.TeamColor ~= vPlayer.TeamColor then end
                                    local dp = dotProduct(normDistance,humanoid.Torso.CFrame.lookVector)
                    tagHumanoid(humanoid, vPlayer)
                    humanoid:TakeDamage(damage)
                    wait(1)
                    untagHumanoid(humanoid)
                    if dp < -0.5 then
                       local m=Instance.new("Message")
                       m.Parent=game.Workspace
                       m.Text="BACKSTAB!!!"
                       wait(2)
                       m:Destroy()
                   end
            end
        end


    end


end