Proves nothing.

Yes it does. When held level the bubble would form at each end of the vial & not centre, If the earth was spherical . Straight vial curved vial or barrel vial. Held at level it proves the earth to be flat.

If the vial is convex upward, why would bubbles form at both ends, regardless of the shape of the earth? The bubble simply floats to the highest point - wherever that is in the vial. One of the "tried & true proven principles" you mentioned earlier.

And that segues right into...

I will ask it again .How do you explain being able to use a level to work out precise degree's of angle.

It's easy. If the vial is shaped as a circular arc, tilting the level rotates the circle that the arc is part of and the bubble moves to the new highest point. Tilting the level by 1 degree will cause the circle to rotate by 1 degree, and the bubble moves 1/360 of the circumference of the circle, or d = 2*pi*r / 360. d is distance the bubble moves, r is the radius of the circular arc, and, just to be clear, * means multiplication and / means division. I presume you know the meaning of pi in this context.

For example, if the radius of the curved vial is 1 meter, a 1 degree tilt would cause the bubble to move:

d = 2 * pi * 1m / 360 = 6.283 m / 360 = 0.0175m, or 1.75 cm. Two degrees would be twice this, etc.

The larger the radius, the more the bubble moves for a given tilt.

Edit: add worked-out example (oops... corrected off by a decimal point in the example).