The vertex algebra $m(1)^+$ and certain affine vertex algebras of level $-1$

We give a coset realization of the vertex operator algebra $M(1)^+$ with central charge $\ell$. We realize $M(1)^+$ as a commutant of certain affine vertex algebras of level $-1$ in the vertex algebra $L_{C_{\ell}^{(1)}}(-\frac12\Lambda_0)\otimes L_{C_{\ell} ^{(1)}}(-\frac{1}{2}\Lambda_0)$. We show that the simple vertex algebra $L_{C_{\ell}^{(1)}}(-\Lambda_0)$ can be (conformally) embedded into $L_{A_{2 \ell -1}^{(1)}}(-\Lambda_0)$ and find the corresponding decomposition. We also study certain coset subalgebras inside $L_{C_{\ell} ^{(1)}}(-\Lambda_0)$.