The paper deals with variational inequalities associated to equations of the type \[ -Lu=f,\qquad f\in L^ \infty(\Omega), \] where \(L\) is a sum of squares of vector fields satisfying a uniform Hörmander condition. The obstacle condition is \(u\geq \psi\). The main result of the paper is a potential type estimate for local solutions, which leads to an upper bound on the oscillation of \(u\) in terms of the Wiener modulus. As an application, Hölder continuity of \(\psi\) at some \(x_ 0\) implies Hölder continuity of \(u\) at the same point.