May work only in object mode. I added a plane and deleted one vertex, added the skin modifier and increased the radius of one vertex.
obj = bpy.data.objects['Plane']
skin_verts = [vertex for vertex in obj.data.skin_vertices[0].data]
each vert in skin_verts will have radius, use_loose, use_root to read and write. Currently radii appear to be represented as 2-float arrays and not as Vector.
>>> for v in obj.data.skin_vertices[0].data:
... print(v.radius[:])
...
(0.25, 0.25)
(0.25, 0.25)
(0.08836718648672104, 0.08836718648672104)
...
writing radii (be in object mode) , each radius has two components which do not need to be the same value ( i just realized that ctrla also lets you shiftx,y the radius )
>>> for v in obj.data.skin_vertices[0].data:
... v.radius = [c*2 for c in v.radius]