Locking a ray on the vertices of a quadrilateral

I have come up with a script for locking a ray onto the vertices of a quadtrilateral, but for some reason it only works about 25 - 50 % of the time, can anyone help me out? (I have already tried using the mesh but that does not work)

function GetCloseToVertice(ObjPos:Vector3, ObjDim:Vector3, PointHit:Vector3)
{
    //var Difference = PointHit - ObjPos;
    for (var u = ((-1*ObjDim.x)/2); u <= ((ObjDim.x)/2); u += ObjDim.x)
    {
        for (var i = ((-1*ObjDim.y)/2); i <= ((ObjDim.y)/2); i += ObjDim.y)
        {
            for (var p = ((-1*ObjDim.z)/2); p <= ((ObjDim.z)/2); p += ObjDim.z)
            {
                if (Vector3.Distance( Vector3( u,i,p)+ObjPos, PointHit) < 0.5 )
                { 
                return (Vector3( u,i,p) + ObjPos);
                }
            }
        }
    }
    return PointHit;
}

Thanks for your answers!