The structure and dynamics of lipid membranes in the presence of extracellular macromolecules are critical for cell membrane functions and many pharmaceutical applications. The pathogen virulence-suppressing end-phosphorylated polyethylene glycol (PEG) triblock copolymer (Pi-ABAPEG) markedly changes the interactions with lipid vesicle membranes and prevents PEG-induced vesicle phase separation in contrast to the unphosphorylated copolymer (ABAPEG). Pi-ABAPEG weakly absorbs on the surface of lipid vesicle membranes and slightly changes the structure of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) unilamellar vesicles at 37 °C, as evidenced by small angle neutron scattering. X-ray reflectivity measurements confirm the weak adsorption of Pi-ABAPEG on DMPC monolayer, resulting in a more compact DMPC monolayer structure. Neutron spin-echo results show that the adsorption of Pi-ABAPEG on DMPC vesicle membranes increases the membrane bending modulus κ.