The problem is considered of the analytical calculation of the fluctuation level of the collective electric field in the case of a collisionless non-dissipative plasma subject to a reactive marginal instability. The existence of this instability only depends on the negative sign of the real part of the dielectric constant e and is not related to the dispersive or dissipative effects described by the frequency dependence or the imaginary part of c. A typical example is given by the non- resonance two streams instability with a symmetric distribution function. In this situation the methods of the thermodynamics of Vlasov systems can be applied to calculate the fluctuation level connected with the instability, provided the Vlasov system is near enough to the marginally stable state and the effect of mode coupling can be neglected (at least for a certain time). In the linear limit, the unstable equilibrium is associated with a minimum of the entropy. When the instability develops new nonlinear terms appear in the dielectric coefficient c, related to the inhomogeneization of the system associated with the growing collective field. These nonlinear terms counterbalance the negative sign of the linear part of c, leading to a saturation of the instability. A neighbouring inhomogeneous equilibrium with long wavelength is then realized, which corresponds to a maximum of the entropy and is stable (under the assumed approximations). The corresponding electrostatic potential is proportional to u−uc, where u is the velocity corresponding to the maxima of the two-stream distribution and uc is the critical value of u for the onset of the instability. The results of the thermodynamic method for the neighbouring equilibria are compared with those obtained by solving the stationary Vlasov equation with conventional analytical methods or with computer calculations.