Vection can elicit a compelling illusion of self-motion, even when the user remains physically stationary. When combined with user actions, this illusion can enhance the sense of presence in virtual environments. However, vection may unintentionally influence physical locomotion, causing unintended positional drift (UPD). UPD is especially problematic for Walk-In-Place (WIP) navigation in virtual reality (VR), where users simulate locomotion by mimicking walking motions without actual displacement. In this study, we investigated how vection affects UPD during WIP navigation in immersive VR. We specifically manipulated the location of peripheral optic flow (left, right, bilateral) and speed (low, high) using dynamic textures on the lateral walls of the virtual environment to induce various vection conditions. Twenty-seven participants performed WIP under six vection conditions (three locations × two speeds) and one baseline condition without optic flow. Our results revealed that bilateral vection caused the greatest UPD, particularly along the lateral axis, while unilateral vection tended to suppress lateral drift. However, peripheral optic flow speed did not significantly affect UPD. We discuss these findings and their implications for designing immersive virtual environments that mitigate UPD while maintaining the sense of presence.