This must be a curse, he thought, and closed his eyes. Vash's singing snapped them right back open though - it wasn't as if he hadn't ever heard the other man sing before, just... under different circumstances.
It was... strange, and he tried to answer normally, hoping the curse was just a one-shot thing, but:
"Now hold this tight don't let go No burden I can't forget the past is real No longer fight this feeling..."