Memorize the melody of the song you want to improve vocal riffs for. Knowing the melody of a song enables you to find the notes that can be used for vocal riffing. In other words, the more you know the melody, the easier it will be to find riffing notes that are in harmony with the song's melody.
Hum your vocal riffs while the song is playing. When you hum a vocal riff that you would like to use on the song, memorize it and get comfortable with its melody.
Add power, vibrato or vocal range to your vocal riff once you have it down. The type of sound you should add to your riff depends on your type of voice. For instance, if you have a great vibrato, adding vibrato to your vocal riff is your safest option.
Add vowels or lyrics to your vocal riff in order to give it more depth. If you aren't using lyrics on your vocal riffs, try different vowels (i.e., eee. aaa, ooo) to see which vowel sounds best with the song.
Record yourself and listen to the playback, which will help you pinpoint if you are following the rhythm and if your riffs match the song's melody.
Memorize the musical queues where you are supposed to sing all your vocal riffs. Although you decide the musical queues of the song, establishing where you are supposed to sing your vocal riffs will help you as a guideline.
Practice the vocal riffs you create regularly. Once you have come up with the vocal riffs you'll be singing on a song, decide how the riffs will be sung, and memorize the musical queues of the riffs. Practice the whole song until you know the riffs by heart. Once singing the vocal riffs becomes second nature, it will be easier for you to find different variations for them, which will help you improvise more comfortably.