Hi Camille, I've been working on my voice and I have this problem. My Voice crack when I try to sing a C5 in my mix voice or in falsetto using the vowel AH(I can overpass the c5 with the rest of the vocals except for that one) And I'm stuck in developing my mix voice because I don't find the way to go higher than a B4. So, What do you recommend for avoid breaking in that C5 using the vowel AH? 

Vowel modification! Try singing an UH first (or another vowel that works for you in that range) and very gradually "morphing" to "AH" - but just until the vowel starts to sound like AH. It doesn't need to be a "pure" AH vowel!