Since $ \begin{pmatrix} a \\ 1 \\ c \end{pmatrix} $ is orthogonal to both $ \begin{pmatrix} 1 \\ - 1 \\ 2 \end{pmatrix} $ and $ \begin{pmatrix} 2 \\ 4 \\ 1 \end{pmatrix}, $ it must be proportional to their cross product: \[\begin{pmatrix} 1 \\ - 1 \\ 2 \end{pmatrix} \times \begin{pmatrix} 2 \\ 4 \\ 1 \end{pmatrix} = \begin{pmatrix} -9 \\ 3 \\ 6 \end{pmatrix}.\]We want the $ y $-coordinate to be 1, so we can divide by 3, to get $ \begin{pmatrix} -3 \\ 1 \\ 2 \end{pmatrix} $. Thus, $ (a,c) = \boxed{(-3,2)} $.