It depends on what you want to acheive and why.
You can't directly include vb.net code in a C# project.
You can access teh vb code FROM your C# project in a couple of ways.
How large is the vb.net page and do you use it in vb.net projects? The easiest way would be to convert it to c# using one of the free converters online.
If you definitely want to keep it as a vb file you can place it in a seperate project and add a reference to that project to your C# project.