Giant unilamellar vesicles (GUVs) are topologically closed, semi-permeable flexible shells (4-6 nm thick, 5-100 µm dia.), isolating femto- to pico-liter quantities of aqueous core from the surrounding bulk. Although water equilibrates across vesicular walls (10-2-10-3 cm3 cm-2s-1), passive permeation of solutes is strongly hindered. Moreover, because of their large volume compressibility (∼109-1010 Nm-2) and area expansion (102-103 mNm-1) moduli, coupled with low bending rigidities (10-19 Nm), vesicular shells bend readily but resist volume compression and tolerate only a limited area expansion (∼5%). Consequently, GUVs experiencing solute concentration gradients exhibit significant shape transformations, deforming in hypertonic media and swelling in hypotonic ones. In this talk, we present experimental evidence, which reveals that, when subject to hypotonic bath, giant vesicles consisting of phase-separating lipid mixtures, exhibit damped, periodic oscillations between a macroscopically phase-separated state and an optically uniform one. This oscillatory phase behavior is synchronized with swell-burst cycles, which characterize lysis: Swelling, caused by the influx of water, raises pressure and membrane tension, promoting the appearance of microscopic domains. Bursting, which enables solute leakage through pore formation, relaxes the membrane tension, breaking up domains producing an optically uniform membrane. This emergent self-regulatory behaviour results from a well-orchestrated cyclical sequence of well-known mechanical processes (tension build-up, phase separation, and poration) induced by osmotic activity of water allowing vesicles to respond (by domain formation) and regulate (by solute efflux) their local environment in a quasi-homeostatic negative feedback loop.