GaAs/InAs/GaAs heterostructures were grown by molecular beam epitaxy on vicinal GaAs (001) substrates. Effects of substrate misorientation on the early stage of InAs epitaxy, as well as the interaction between InAs and a GaAs overlayer, were studied by transmission and scanning electron microscopies and by photoluminescence measurements. The formation of InAs islands were observed after a few monolayer InAs deposition. Two major results were obtained in this study : (a) Upon deposition of a crystalline GaAs overlayer, InAs islands undergo a novel type of morphological transition, i.e., from disk-shaped to ring-shaped ones. (b) Substrate misorientation results in anisotropic effects on InAs island formation. In comparison with on-axis or [110] tilted samples, substrate misorientation toward [110] by up to 5-degrees leads not only to reduction in InAs island density by a factor of 2, but also to the formation of InAs quantum dots. These results were found to be consistent with photoluminescence experiments.