quit leaving no vowels at the end of the game.
Reply from Rick: The letters and their positions are decided randomly at the beginning of the game. Part of the challenge is figuring out how to remove them all and leave yourself some vowels left so you can get rid of all of the tiles.
One strategy I use is to try and leave as many wild cards and vowels that by themselves (A and I) make a word at lowest layer of the layout. If you use them as late in the game as possible, you can always use those tiles to make a single letter word at the end of the game.
Hope that helps, :-)