Write a function that replaces all vowels in a string with a specified vowel.
vow_replace("apples and bananas", "u") ➞ "upplus und bununus"
vow_replace("cheese casserole", "o") ➞ "chooso cossorolo"
vow_replace("stuffed jalapeno poppers", "e") ➞ "steffed jelepene peppers"