The next three warmups in my sequence are all vowels or vowel combinations. I usually begin on low A and do the arpeggio pattern, 1-5-3-8-5-3-1, or do-sol-mi-do-sol-mi-do, up to about high A. You should drop out, switch to falsetto or head voice, or drop down an octave if the exercise begins to feel tight or if you feel discomfort. Make sure to sing legato.
3. The vowel EE or [i] in IPA.
4. The vowel combination EE-AY or [i]-[ɛI] in IPA. (Feel free to drop the diphthong.)
5. The vowel combination AY-AH or [ɛI]-[ɑ] in IPA.
Here is a link to an mp3 I made to use as a background in order to practice.