In this study, a plant extract-based functional synthetic medium was developed to support the growth of kimchi-derived lactic acid bacteria (LAB) for membrane vesicle (MV)-based postbiotics. Four LAB strains [Leuconostoc mesenteroides, Lactiplantibacillus plantarum, Latilactobacillus curvatus (Lc), and Latilactobacillus sakei] were fermented for 3 days in artificial media containing various plant-derived extracts, including barley sprout extract, baechu (kimchi cabbage) extract, and a plant mixed extract (Platycodon grandiflorum: Codonopsis lanceolata: sweet potato peel=1:1:3). Growth was more vigorous in media with barley sprout extract. Notably, Lc showed significantly enhanced growth when the plant mixed extract was added, indicating a positive effect of this mixture on Lc proliferation. Fermented samples exhibited increased LAB counts and elevated digestive enzyme activities (trypsin and α-amylase) compared to non-fermented controls. These enhancements correlated closely with increased total polyphenol and flavonoid contents, which significantly rose after fermentation, especially in Lc-inoculated groups. Moreover, DPPH radical scavenging activity was markedly higher in Lc-fermented samples than in those fermented by other strains, highlighting Lc’s efficacy in producing antioxidant compounds. This study demonstrates that barley sprout extract is a viable medium for developing MV-based postbiotics, with strain selection and fermentation optimization being critical for functional enhancement.