How do i repeat an if statement until it's false in Java

So i have this code that is supposed to generate random words of length the user decides with these conditions:

[1] Every other letter is a random vowel (letter + vowel + letter + vowel...etc)
[2] Two vowels cannot cannot meet together (ie: bout, caravaa)

I got over the first part condition, and the second condition needs some edit. i used an if statement to check if letter1 or letter2 happen to be vowels, and if true generate generate different letters. Problem is those newly generated letters have a chance of being vowels. How do i keep repeating the if statement until it's false? (Letter1 and Letter2 are not vowels)

import java.util.Random;

public class NickName_1
{
     public static void main(String[] args)
{
    java.util.Scanner scanner = new java.util.Scanner(System.in);

    int letterNum = 6;
    Random rn = new Random();
    char letter1 = (char) (rn.nextInt(26) + 'a');
    char letter2 = (char) (rn.nextInt(26) + 'a');
    char[] vowels = { 'a', 'e', 'i', 'o', 'u' };
    char vowel;


    while (true)
    {
        //Ask user for word's length and store it
        System.out.println();
        System.out.print("Please enter the length of the word in letters: ");
        letterNum = scanner.nextInt();

        vowel = vowels[(int)(Math.random() * 5)];

        for (int i = 0; i < (letterNum / 3); i++)
        {
            letter1 = (char) (rn.nextInt(26) + 'a');
            letter2 = (char) (rn.nextInt(26) + 'a');
            vowel = vowels[(int)(Math.random() * 5)];

            // if letter1 or letter2 equal a vowel then regenerate letters
            if (letter1 == 'a' || letter1 == 'u' || letter1 == 'i' || letter1 == 'e' || letter1 == 'o' || 
                letter2 == 'a' || letter2 == 'u' || letter2 == 'i' || letter2 == 'e' || letter2 == 'o')

            {
                letter1 = (char) (rn.nextInt(26) + 'a');
                letter2 = (char) (rn.nextInt(26) + 'a');
            }
                System.out.print(letter1 + "" + vowel + "" + letter2);
        }
    }
}
}