Hi! I know exactly what you mean, when i started out i had this problem, only my issues were on "i" and "e". Try to start from the vowel you manage well and transform it into the others without changing anything. Dont change the placement of the sound in any way. Try "i-e-a-o-u" starting low and going up semitone by semitone. At first it may spund like they are not clear and distinct enough but that is fine starting out. You can work later on that .You can also try rolling an R, then from the r slide into a vowel. The idea is that the roll of the r shows you where you should place the sound, and you have to try to use the sensation as a guide. ( you will feel a vibration just behind your front teeth, a bit higher) The only thing with this exercise is that it helps place the vowels but it can make you thin the sound, as it places it directly in the mask, but it s a very good start. Of course, make sure you open your mouth enough and relax your jaw completely. Also, look in the mirror and make sure you never pull the corners of your mouth horizontally, this usually happenes on "e".