  We calculate the stress tensor for a quasi-spherical vesicle and we thermally
average it in order to obtain the actual, mechanical, surface tension $\tau$ of
the vesicle. Both closed and poked vesicles are considered. We recover our
results for $\tau$ by differentiating the free-energy with respect to the
proper projected area. We show that $\tau$ may become negative well before the
transition to oblate shapes and that it may reach quite large negative values
in the case of small vesicles. This implies that spherical vesicles may have an
inner pressure lower than the outer one.
