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 $L_{C_{\ell}
^{(1)}}(-\tfrac{1}{2}\Lambda_0) \otimes L_{C_{\ell}
^{(1)}}(-\tfrac{1}{2}\Lambda_0)$. 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).