Hi MadhuG001,
Vb.net should suffice but knowing C# can be a "good to have" if not a "must have". Like anything in technology, the more you know the better, easier and comfortable it gets. Jack of all and master of none has become the norm. You can open the existing in-built VBOs in Blue Prism and see that a majority of them are written using Visual Basic as the set language in their Code stage. But the Email - POP3/SMTP VBO is written in C#. So if one knows Vb.net there can arise a possibility to understand the code of the Email VBO and make modifications to it if it ever becomes necessary. Also some organizations stick to using one language in any new custom made VBOs. So in that case if the preferred language is C# then it is better to know it rather than struggling to read/write its code.
Another thing to add is if your work involves working with Office applications then it is good to know the Object Model of those applications like Excel, Word, etc. This is irrespective of Vb.net or C# because even if you know either or both of these languages one needs to understand the Object Model of these Office applications in order to interact with them.
For example what are the Range, Cells, Sheet object in Excel, how to interact with a Pivot object, what is the Document, Paragraph and Table object in Word. Even though if your work does not involve these right now, it is good for later purposes since spreadsheets (and documents) are widely used in automation as most of the manual work is/was happening in spreadsheets.