We report an investigation on the interaction of the positively charged surfactant DDAB (didodecyldimethyl-ammonium bromide) with preformed vesicles from POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine). Addition of 1.9 mM DDAB to preformed POPC vesicles of different concentrations results in mixed vesicles, owing to the avid uptake of DDAB by POPC. This leads to the so-called matrix effect: when preformed POPC liposomes with a narrow size distribution are present in the aqueous solution, there is a fast formation process of mixed vesicles and the final size distribution closely resembles that of the preformed POPC liposomes. The final mixed vesicle system is stable in size and size distribution. The effect is independent of the initial size of the POPC vesicles and requires low relative concentrations of POPC with respect to DDAB (up to 1:4). Aside this, we report about the time stability of DDAB vesicles as a function of the aqueous buffer ionic strength and we investigate the interaction between negatively charged surfactant oleate and DDAB. The formation of mixed vesicles is observed, whose size distribution is strongly dependent on the mixing, ratio of the two surfactants. Interestingly, the mixing of DDAB and oleate vesicles for values of DDAB molar fraction close to 0.4 leads to a very narrowly distributed vesicular species centered around 100 nm.