VB and C# are quite similar in .NET, i.e VB is a full member of the .NET platform. I cannot see any reason that force You to learn C#. The only major difference between them is that C# can be more used for advanced plumbing. If You use an earlier version of VS then 2010 then VB is the preferable language to use as with C# You must always declare all parameters in Excel's objects/method/properties.
If You have been working with VBA then it will be easier to port the code to VB then to C#.