Round or hard corners on a single vertices in 2D I have an 2D object (eg. circle) and would like to modify single vertices and their angle. Very much like in any graphic software I would like to select one single vertice and convert it either in a hard corner or a "soft"/smooth corner.
Couldn't find that in Blender. Proportional editing and smoothing won't work. 

Related to this question I would like to manipulate vertices by exact dimensions and find that sometimes this is hard to achieve. For example I would like to scale by an exact dimension.
Mesh objects always have what you seem to be thinking of as "hard corners". When you're saying "like in any graphic software" I assume you're thinking about vector-based software like Inkscape or Illustrator which typically use Bezier curves.

If you want to manipulate points in a similar manner you would have to create Curve objects, not Mesh objects.