As with anything that flies, there is NO single attribute that MUST be "thus-and-so".. Not wing area, weight, power, surface configuration, c.g. whatever.... Any airplane is a series of design compromises, all intended for a specific purpose, which is why they look different, when the purposes are different.
For the current instance, which involves tail areas and angles, the areas for the tail itself can vary easily from least acceptable to more acceptable by a factor of 2 or so, and the aircraft will be flyable for whatever purpose it is sized for, when other areas of the design (compromises) are included in the design.
And if the areas are too small/large, other accomodations (compromises) in other parts of the design can be utilized.
It may be better to fix too-small/too-large, but neither need be fatal. A compromise can be designed.
Nor are they "required" or "best"..
As the areas can vary, so can the angle between the surfaces of a vee-tail..
A steep (90°) seperation between vee'd surfaces could be selected for a rudder-elevator plane which will be turning a lot.. as a sloper, for instance. The turning quality overrules any other quality which might be present in the configuration.
(Turning quickly is the most important attribute a sloper can have.)
The same identical airplane as a thermal duration flier could use a more relaxed angle.. greater than 90°, as the flight requirements are different.. rapid response is NOT a prime flight consideration; ease of control and stable flight is.
An extreme relaxed angle, (135°) on an aileron, vee-tail elevator airplane, again as a sloper, makes use of the ailerons for the turning quality, and the relaxed angle on the elevator for more pitch authority.
Such planes make excellent flat-spinners for one thing.
Anywhere in between is useable for the intended purpose, AND the control configuration of the airplane.
The angles seen on thermal gliders are there because they work well, for thermal gliders.
An aerobat would go for something different.
A pylon racer....
All these design qualities depends (compromise) on what manuvers will be intended for THAT airplane.