The Riemann problem admitting δ-,<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"><mml:msup><mml:mi>δ</mml:mi><mml:mo>′</mml:mo></mml:msup></mml:math>-shocks, and vacuum states (the vanishing viscosity approach)