Minimum and Maximum values
When mapping vowels to an effect it can be a good idea to set the minimum and maximum value of that parameter in your DAW. This can give you tighter control over the effect when using vowel sounds.
Mapping to Volume
Try mapping the envelope knob to the volume level on a channel. This can give the effect of rising intensity with the voice.
Although the knobs of the vowels scale up and down, they can also be used to trigger on/off settings in your DAW. For example, try activating an arpeggiator when you use an ‘aaaa’ vowel.