Korean beauty trends, amplified by TikTok, have positioned Vietnam as a critical cultural intersection where digital aesthetics converge with evolving consumer identities. Despite this phenomenon, the specific mechanisms by which this digital fascination translates into brand loyalty and purchasing behavior remain insufficiently theorized. Grounded in the Theory of Planned Behavior (TPB), this study investigates how social media exposure influences Vietnamese consumers’ purchase intentions toward Korean cosmetics. Specifically, it examines the mediating roles of trend adoption-conceptualized as a normative route reflecting transient social conformity and brand awareness, representing an attitudinal route rooted in cognitive trust. Using survey data from 199 respondents, the findings indicate that while social media exposure positively impacts both mediators, brand awareness exerts a significantly more robust influence on purchase intention. Furthermore, cultural affinity was found to moderate these relationships, amplifying the brand awareness-to-intention pathway and the indirect effect of social media exposure. Conversely, cultural affinity exerted limited influence on the trend adoption route. Theoretically, this research extends the TPB by integrating cultural affinity as a dynamic boundary condition and disentangling normative versus attitudinal processes in cross-cultural digital consumption. Practically, the results suggest that in high-affinity markets, long-term consumer loyalty is better sustained through culturally embedded brand narratives rather than ephemeral, trend-based campaigns.