An EASA ATPL question/answer insists, counterintuitively, that a twin which loses an engine and must drift down will find its least rate of sink at Vxse. I've been trying to understand this, but haven't come up with an answer yet. Do Vy and Vx even exist as concepts when above the ceiling? Does "best rate of climb" become instead "least rate of descent"? Does "greatest distance between the power-required and power-available curves" become "least distance" between same? Would the speeds of Vx and Vy (with or without SE) change when going from below ceiling to above while at the same altitude and mass?
One way to determine Vx and Vy is to look at a ROC vs Speed chart, where ROC will be an inverted parabola. The peak of the parabola is Vy, the tangent of it to the origin is Vx. If the whole curve is shifted below the x-axis, however, the tangent will fall on the far side of the Vy peak. So I would expect Vx > Vy when above the ceiling. But that still doesn't help understand why EASA would say least sink is at Vxse.