STEM students often lack oral communication confidence despite strong technical skills. We present PAVE (Performance-Adaptive Virtual Environment), a novel VR-based public speaking trainer that heightens evaluation pressure through real-time, performance-driven audience adaptation. Unlike prior systems, PAVE uses 34 virtual avatars whose facial expressions shift every 30 s based on four live behavioral metrics: movement, gestures, gaze, and filler word use. This exaggerated, synchronized feedback is designed not to mimic realism, but to induce evaluation apprehension and support self-efficacy through emotionally salient mastery experiences. In a 2 \(\times \) 2 mixed-design study with 20 STEM students, PAVE significantly increased willingness to communicate and social presence. Our findings underscore the potential of adaptive, affective VR systems for building communication confidence in technical education.