In this work we investigate the influence of the Si substrate orientation on the growth instability of strained Si1-xGex heterostructures. The work mainly consists in atomic force microscopy and grazing incidence x-ray diffraction analyses of the Si1-xGex layers deposited by gas source molecular beam epitaxy on vicinal Si substrates tilted from (001) to (111) surfaces. The major result is that the two- to three-dimensional growth transition is dramatically affected by the orientation of the substrate but also by the equilibrium shape of silicon. For instance, we evidence the layer by layer growth of Si1-xGex on Si (111) in contrast to the nucleation of three-dimensional islands on 2 degrees off Si (111) in the same experimental conditions. We systematically verify that the homoepitaxial growth of unstressed Si on vicinal Si (111) consists in a regular array of single steps. Therefore, we propose that the stress induced by the heteroepitaxial growth destabilizes the regular step train by reducing the repulsive elastic interaction between steps, and induces step-bunching The presence of close-spaced steps and the metastability of the vicinal surfaces increase the tendency towards instable growth and result to earlier development of bunching. Despite the accompanying increase of surface area, the development of low-energy facets balances the surface free energy excess. In all cases, step-bunching instability is a kinetic pathway towards the faceted equilibrium state. Long annealing treatment of the strained metastable Si1-xGex layers confirms this last point.