We get very small steps (1e-15) from the Cerenkov process in the VMC tests and so we found out that there is a problem with setting the MaxBetaChange parameter, where a unit perCent is added in G4Cerenkov::SetMaxBetaChangePerStep() and the updated value is passed back to G4OpticalParameters.

This method is however call at the initialization phase for all particles via call to Initialise() called by G4Cerenkov::PreparePhysicsTable():
  SetMaxBetaChangePerStep(params->GetCerenkovMaxBetaChange());

With each call, the perCent is applied again, and the final value is of order 1e-106. The control printing added in G4Cerenkov::PreparePhysicsTable():

   Initialise();
+  // control printing
+  G4cout << "Initialise for " << particle.GetParticleName() << G4endl;
+  G4cout << "fMaxBetaChange: " << fMaxBetaChange << G4endl;

gives:

    Initialise for B+
    fMaxBetaChange: 0.01
    Initialise for B-
    fMaxBetaChange: 0.0001
    ...
    Initialise for xi_c+
    fMaxBetaChange: 1e-106

To fix this, the unit should be applied in the cerenkov/setMaxBetaChange command and removed from the setter in Cerenkov process.