C++ is a very different beast to C#, even if it bares some resemblance to it via syntax.  C++ is a very difficult language to master.  It's one thing to be able to read it, it's another to write it and write it effectively.  It is much, much easier to go from C++ to C# than it is the other way around.

You need to ask yourself why you need to work in C++.  C++ as a development language is used for very specific reasons(performance is the chief one).  Use the right language for the job, don't try to shoehorn in one language or another simply because you know the language or you want to learn a different one.

I would not say C++ is a encountering any sort of renaissance.  It's more that, despite the last decade or so of VB, C# and Java, where at many points in the past it's been popular to state "C++ is dead", C++ is very much alive and well.  There simply is no language out there that will replace C++ for certain applications or pieces of certain applications. Microsoft recognizes this fact and thus is smart in ensuring that C++ continues to be a first class citizen in the suite of Windows development tools.

I will say that to know C++(eh, rather master it), in addition to C#, will open up many more opportunities for you in your career.  There is a much smaller pool of C++ experts out there.  And in the recent past C++ has been dropped as a requirement for graduation by many CS university programs, further constraining the supply of good C++ programmers.