If C# and VB Pages in same Project then you can access it by session name but if you have added Different Project in same Solution for VB and C# then you need to use Session state Mode as SQLServer mode.

Refer the below links for your reference.

https://msdn.microsoft.com/en-us/library/ms178586.aspx

How to share session between two applications

Sharing ASP.NET Web Application's Session State

Sharing session value between two different asp.net solutions using "SQL Server" mode

Exploring Session in ASP.NET