A simple model for amphiphilic molecules is sufficient to predict self-assembly into spherical vesicles. The model is also useful to study the influence of osmotic pressure oil the shape of vesicles, which serve as carriers in biological cells. The stability of small vesicles subjected to relatively high osmotic pressures is demonstrated. Fluctuations of small vesicles under osmotic stress are found to be in semiquantitative agreement with it macroscopic description. Small deviations between simulated results for vesicle fluctuations and macroscopic elasticity theory could result from the relative large membrane thickness in comparison to the vesicle radius. The simulations demonstrate that an osmotic pressure exerted by solute molecules outside a spherical vesicle can cause a shape transition, in agreement with results based on the elasticity theory of membranes.