Photoluminescence (PL), reflectivity and thermally-detected optical absorption (TDOA) experiments have been carried out, at liquid helium temperatures, On In0.35Ga0.65As/GaAs quantum wells (QWs) with different thicknesses of 4, 6, 8 and 10 monolayers (MLs), grown by molecular beam epitaxy (MBE) on substrates oriented exactly on (100) (nominal growth) or misoriented by 4-degrees or 6-degrees towards (111)Ga (vicinal growth). Indium segregation effects are considered in the comparison of die calculated fundamental (e1hh1) transition energies of the QWs with the experimental results. Within the framework of a simple model, a segregation energy of 0.35 eV is used to fit the experimental data. It is shown that the misorientation improves the optical properties of QWs: diminution of the spectral broadenings and reduction of the blue shift between reflectivity structures or TDOA peaks and luminescence lines. This suggests a better quality of the interfaces and an improvement of the strain homogeneity. A step edge effect on the fundamental transition energy of the 6 and 8 ML quantum wells is shown but the blue shift observed for a growth on a 4-degrees off misoriented surface can also be due to a modification of the growth mode.