Given enough thrust, a jet-powered aircraft could reach ‘coffin corner’, an altitude where controlled level flight is only possible at one speed. This altitude is the ‘absolute ceiling’.
At an altitude just below the absolute ceiling, flight at different speeds is possible but the available range of speeds will be small. A typical definition is that the ‘service ceiling’ is that where a rate of climb of 100fpm is still possible. It follows, therefore, that Vy and Vx will converge as altitude increases, eventually meeting at the absolute ceiling.
Sorry Vessbot, I think I crossed with your post.