There is no "up" in vectorized geometry.
There is an angle, and a distance from the origin. But there is no up.
Otherwise, show me how you add "up" with the vector AC = (-1; 5; 5) relatively to the origin.
Congratulations on your pedantic argument. Is it really that hard to read posts and understand what this word intends from the context?
"Up" is generally accepted to be the direction perpendicular to Earth's surface at a particular location, and that is the sense in which it is being used here. More specifically, it is the direction opposite the acceleration that we feel towards Earth. If you wish to formalize the notion of "up" in a standard 3-dimensional Cartesian coordinate system, you can just think of it as a unit vector pointing parallel to the z-axis (where the x and y axes are horizontal, or found within the plane of the ground).
In the specific context of this thread, up is defined as the direction in which UA is proposed to be accelerating the Earth. Because you propose that gravity is nonexistent in your models, UA necessarily provides the acceleration towards the Earth that we feel. The Ferguson model necessarily fails because the water on the planet would see a component of this acceleration directed towards the low point in the bowl, hence momentia's argument that some parts of the world would be submerged on that model.
If Universal Acceleration is responsible for what we conventionally call gravity, then the Earth must be flat, and exist on a plane perpendicular to the direction in which UA is accelerating it. If you wish to hypothesize that the Earth takes on a more exotic shape such as that in the Ferguson model, you must reject UA and either construct a model in which gravity explains our tendency to accelerate towards Earth or construct a model using a different explanation entirely.