"You should practice technique for ten minutes at a time, five times a day. Then practice repertoire. That's how it's done!"
Margaret Harshaw taught me this approach more than twenty years ago. How interesting then to come upon research that supports her perspective, which you can read here—the essence of which is short bursts of practice with an emphasis on randomness. What might this look like?
Let's say you are practicing the /i/ vowel.
Start with calling on /i/.
Then sing /i/ on a single long tone.
Then sing /i/ on a 5 tone scale.
Go back to calling /i/.
Mix & Match.
Then you could do the same thing with with /i/, /e/ and /a/. The permutations are as endless as they are interesting. This approach to practice follows "The Ten Minute Rule," which takes into account that if you are practicing technical exercises longer than ten minutes by yourself your brain is going to go dead! It's nice to have science agree.