Usually one thinks of the symplectic structure $\omega=d\lambda$ as living on the cotangent bundle $T^*N$ of a manifold $N$, where it is the exterior derivative of the Lioville 1-form. Hence, contracting it with a vectorfield will require that the vectorfield lives on $T^*N$ and the result will be a 1-form on $T^*N$. Since your Lagrangian $L$ lives on $TN$, $dL$ is a 1-form on $TN$ so something needs to be corrected.

The way one usually looks at this is the following: If the Lagrangian $L$ on $TN$ is Tonelli then it gives rise to a Legendre transformation $$ l:TN \to T^*N, $$ which is a diffeomorphism. This allows one to transfer the dynamics of $L$ to the cotangent bundle. The way this is done is by defining a Hamiltonian function $H:T^*N \to \mathbb{R}$ via $$H(l(q,v))=\partial_vL(q,v)\cdot v-L(q,v), $$
where $\partial_vL(q,v)$ denotes the fiber derivative of $L$ and $(q,v)$ are coordinates on $TL$, $q$ being the base-coordinate and $v$ the fiber-coordinate. The Hamiltonian flow of $H$ is then defined exactly as you say: Namely it is the flow generated by the vectorfield $X_H$ whose contraction with $\omega$ equals $dH$. The vectorfield $X_L$ on $TN$ which generates the dynamics dictated by $L$ is now obtained by $$dl \cdot X_L=X_H.$$ Of course one can also derive the fomula for $X_L$ using the principle of least action.

Deriving all the above is a bit tedious, but it is carried out in great detail in the first chapter of Mazzucchelli's book "Critical Point Theory for Lagrangian Systems" http://www.springer.com/gp/book/9783034801621.

Usually one thinks of the symplectic structure $\omega=d\lambda$ as living on the cotangent bundle $T^*N$ of a manifold $N$, where it is the exterior derivative of the Lioville 1-form $\lambda$. Hence, contracting it with a vectorfield will require that the vectorfield lives on $T^*N$ and the result will be a 1-form on $T^*N$. Since your Lagrangian $L$ lives on $TN$, $dL$ is a 1-form on $TN$ so something needs to be corrected.

The way one usually looks at this is the following: If the Lagrangian $L$ on $TN$ is Tonelli then it gives rise to a Legendre transformation $$ l:TN \to T^*N, $$ which is a diffeomorphism. This allows one to transfer the dynamics of $L$ to the cotangent bundle. The way this is done is by defining a Hamiltonian function $H:T^*N \to \mathbb{R}$ via $$H(l(q,v))=\partial_vL(q,v)\cdot v-L(q,v), $$
where $\partial_vL(q,v)$ denotes the fiber derivative of $L$ and $(q,v)$ are coordinates on $TL$, $q$ being the base-coordinate and $v$ the fiber-coordinate. The Hamiltonian flow of $H$ is then defined exactly as you say: Namely it is the flow generated by the vectorfield $X_H$ whose contraction with $\omega$ equals $dH$. The vectorfield $X_L$ on $TN$ which generates the dynamics dictated by $L$ is now obtained by $$dl \cdot X_L=X_H.$$ Of course one can also derive the fomula for $X_L$ using the principle of least action.

Deriving all the above is a bit tedious, but it is carried out in great detail in the first chapter of Mazzucchelli's book "Critical Point Theory for Lagrangian Systems" http://www.springer.com/gp/book/9783034801621.