a language is a tool, not a destination.
c# is built on the .net framework. it provides the same power as vb.net and all the same framework components.
what is nice about these languages is that they are very highly powered, high level, rapid development platforms.
the last part is the key. rapid development platforms.
c++ will give you more horsepower (that you don't really need for bot design), and allow you things like more granular access to the way memory is allocated, etc...
it's not really a rapid development platform.
for my money, vb.net is where it's at. the syntax is easy to learn and understand, and you get all the power and benefits of using the .net framework.
c++ for bots would probably be a bit over kill. that's not to say don't ever bother learning. as a developer the more languages you know the better, but you will probably find it easier to learn c# or vb.net, and will probably be able to start producing valuable results sooner.
one thing also that bears mentioning. .net has some great support in their framework for working with the internet. things like httpwebrequest and the sockets namespace allow you to easily make powerful internet connected applications.