public partial class MainPage : PhoneApplicationPage
{
    private const CameraSensorLocation SENSOR_LOCATION = CameraSensorLocation.Back;

    private AudioVideoCaptureDevice _videoDevice = null;

    // ... ctor and camInitialization methods etc.

    private async void CameraButtons_ShutterKeyPressed(object sender, EventArgs e)
    {
        try{
            StorageFolder folder = await ApplicationData.Current.LocalFolder.CreateFolderAsync("videos", CreationCollisionOption.ReplaceExisting);
            StorageFile storageFile = await folder.CreateFileAsync("Video.mp4", CreationCollisionOption.GenerateUniqueName);

            using (var s = await storageFile.OpenAsync(FileAccessMode.ReadWrite))
            {
                _videoDevice.VideoEncodingFormat = CameraCaptureVideoFormat.H264;
                await _videoDevice.StartRecordingToStreamAsync(s);

                Thread.Sleep(3000);

                // I want  the following 3 lines to be executed when I press the button again
                await _videoDevice.StopRecordingAsync();
                await s.FlushAsync();
                s.AsStream().Dispose();
            }
        } catch(Exception ex){
        }
    }
}