I'm a beginner. My challenge is identifying and distinguishing the vowels in the first place, not academically analyzing them. I went to print off a vowel chart for reference, and I found some variations.
Wikipedia has this chart, which has each of 15 vowels in their own locations, with notes that two pairs may be merged.
One can also find simplified vowel charts with as few as 9 locations, merging the locations of e.g. /i/ and /y/, as in the chart below
This seems better to me from a beginner perspective...I already think that /i/ and /y/ have the same location but with different rounding, even though it may not be exactly correct, it's my current working model. Then again, some of the positions are really different between the charts. Is there harm in using the simplified version when starting out?