Well usually I just chalk it up to Vhagar loving Aemond so much that she just hated Lucerys more than him and wanted him dead herself. She roars as soon as Lucerys' foot touches the ground on Storm's End, she hates that boy so much. I just don't put it past her. Also, he was telling her stop in english because I think he was too panicked to think to say it in Valyrian, and she's a war dragon, let a girl have some fun and relive her glory days, damn. She loves her unhinged dragonrider, and I know she's the one encouraging him to become more unhinged, trust me. She loves her little war criminal.