Dear @Aengus.Connolly,
By “vibrating at high frequency”, can you confirm which frequency this is? I would guess the frequency corresponds to the blade edgewise-bending natural frequency. This would suggest that the issue you are facing is the classic edgewise instability brought about by negative damping introduced in deep stall, e.g., as discussed in the following forum topic: Designing for yaw errors using FAST.
Best regards,