But you are right: The higher you go, the closer Vx and Vy will become. It's just that for various practical reasons you will never be able to actually fly at the point where Vx and Vy intersect. (Unless you have infinite fuel and infinite patience I guess.)
One can easily fly above the published service ceiling, and easily above the absolute ceiling for the airplane. One only needs a source of lift.
I've flown Cessna 150's well above the published altitudes; in fact I've flown them to what used to be the base of the Continental Control Area (14,500' in the USA), and to what's now Class A airspace (18,000 in the USA), or formerly "Positive Controlled Airspace." Convective lifting action and oxygen, and a few hours to kill over Kansas. No problem. It was a part of an experiment, and a successful one.
Bear in mind that while service ceiling is defined by certification standards (max sustainable 100 fpm in single engine airplane, and 50 fpm for multi engine airplanes on one engine for single engine service ceiling), absolute ceiling is the point where the airplane can no longer climb under it's own power. This is an environmental question, given temperature and leaning of the piston engine.
Various other types of ceilings also apply.