How to connect two vertices with curved edge Is there a way to do something like this


from this?


I've created the first one by adding circle and removing unwanted parts, but what if I couldn't start from scratch and wanted to create it out of two vertices only? Thanks
As Leander has commented, if you want a circular arc, you can use the Spin tool, (AltR) to spin a single radius around the 3D cursor. 

If you want a more arbitrary curve interpolating through control points, you can select and position those points on an existing boundary, and use the shipped add-on: Loop Tools > 'Curve' to form it.