Serialize a list of subtitles according to the VTT format, with optional time padding.

def vtt_formatter(subtitles, padding_before=0, padding_after=0):
    """
    Serialize a list of subtitles according to the VTT format, with optional time padding.
    """
    text = srt_formatter(subtitles, padding_before, padding_after)
    text = 'WEBVTT\n\n' + text.replace(',', '.')
    return text