Sajid,
You can't, really, as you would need to host the scripting runtime in
your program, and to be quite honest, there isn't anything in the VBScript
language that you can't do in C# (which I am not aware of).
What you might want to do is add a reference to
Microsoft.VisualBasic.dll, which contains most of the functions (they are
static) that VB.NET exposes. VBScript is a subset of VB (the old VB5 and
VB6) and VB.NET didn't change drastically in terms of intrinsic functions
which it offers (if at all), so you could just make the calls directly.
Or, are you referring to a script file that you wrote? If this is the
case, you will have to use the Process class to execute the file. However,
getting the results back would be tricky (as well as passing parameters in).
You really should consider migrating that code to a class and exposing it
that way (if anything, you can expose the function through COM interop and
have the script access your COM component, and your .NET code access the
same component).