That's a pretty bad comparison. The differences between VB and C# are much more dramatic than the differences between Python 2 and Python 3.
C# takes it much further, and iterates on syntax a lot more, but Python3's breakage from Python2's is comparable I feel, just not in nearly as extreme a case.
They're not. You can create "single source" Python packages working on both Python 2 and Python 3. Good luck doing that with C# and VB.