The dot net framework 3.0 added the system.speech namespace.  One of the new classes they added is the SpeechRecognizer.  The speech recognizer class has a SpeechRecognized event which you can use to make your application accept dictation.

For this example you need to create a .net 3.0 or .net 3.5 windows forms app.  Place a multi-line textbox on the form. 

Imports System.Speech
Imports System.Speech.Recognition

Public Class Form1
    Dim recognizer As New SpeechRecognizer

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        AddHandler recognizer.SpeechRecognized, AddressOf SpeechRecognized
    End Sub

    Private Sub SpeechRecognized(ByVal sender As Object, ByVal e As SpeechRecognizedEventArgs)
        TextBox1.Text &= e.Result.Text
    End Sub
End Class