> My Call Center Applicaton would communicate with heavy duty PBX systems, I thought the best result (the best application performance in heavy tasks conditions)

Application performance in this case does not depend on the language. It depends on the design and the skills of the people involved. Depending on how you communicate with the PBX systems this might be better done in C++ or C#. But on any of the higher levels you don't gain anything with C# over VB.NET. I wouldn't worry about the language here, just pick what feels most comfortable. Rather spend the time on creating a design that allows for performance, ie. is asynchronous, queue driven, has clearly separate tiers, etc.