If the strings are defined in the same order as in your question, you can avoid sorting, and here is a simple VB.NET answer:

Public Function Extract() As String
    Dim type As Type = Assembly.LoadFrom("C:\test.dll").GetType("YourNamespace.Test")
    Dim instance As Object = Activator.CreateInstance(type)
    Dim sb As New StringBuilder
    Dim field As FieldInfo
    For Each field In type.GetFields
        If field.Name.StartsWith("Prefix_") Then
            sb.Append(field.GetValue(instance))
        End If
    Next
    Return sb.ToString
End Function

otherwise here is a function with sorting:

Public Function Extract() As String
    Dim type As Type = Assembly.LoadFrom("c:\test.dll").GetType("YourNamespace.Test")
    Dim fields As New List(Of FieldInfo)
    Dim field As FieldInfo
    For Each field In type.GetFields
        If field.Name.StartsWith("Prefix_") Then
            fields.Add(field)
        End If
    Next

    fields.Sort(New FieldComparer)

    Dim sb As New StringBuilder
    Dim instance As Object = Activator.CreateInstance(type)
    For Each field In fields
        sb.Append(field.GetValue(instance))
    Next
    Return sb.ToString
End Function

Private Class FieldComparer
    Implements IComparer(Of FieldInfo)

    Public Function Compare(ByVal x As FieldInfo, ByVal y As FieldInfo) As Integer Implements IComparer(Of FieldInfo).Compare
        Return x.Name.CompareTo(y.Name)
    End Function
End Class