A career in software development tends to be a lot more fluid when it comes to technological knowledge than just "I am using language X" - I have worked in computer security (C++, Java, Perl, Python), defense (C++), and some startups and web companies (Java, Python, C#, Ruby). I have never really been afforded the opportunity to be a single platform or single-language person, except at my current job (using C# in a pure Windows shop), and Visual C++ is a specific implementation of the C++ language that is only available on a single platform (Windows, though the C++ knowledge itself is rather portable in terms of language constructs).
More than likely, you will be learning and using multiple technologies, languages, and platforms to stay up with the speed of the Valley. I interviewed out there a few months ago (at a very nice startup, got an offer but ultimately turned it down due to personal and family obligations here). They were a Ruby company (I wanted a Ruby job but had never used Ruby professionally) and did not care that I had not use Ruby professionally. Companies out there are not going to be interviewing you for very specific knowledge, they need to find someone who's very flexible, adaptable, and able to pick up many things and do multiple things in rapid order. Windows itself seems to be a rather rare development platform in Valley and Valley-ish companies - developers tend to use Mac OSX or Linux, and companies use Linux for servers (due to cost and general availability on platforms like Amazon EC2, Heroku, and Engine Yard).
I will say the most important thing is to keep experimenting and learning - even on your current job. I am developing my Python, Scala, and R knowledge by using them at my current job (the company does not use any of them in production) because it's easier for me to prototype and experiment in those technologies than using C# up front. It helps me get work done faster in the end. Also, understand that there are many technologies out there and while you will develop expertise in a few, you will gain passing knowledge across a wide variety of technologies, platforms, and languages.