Roundedness
In pronouncing these vowels, your tongue can’t do it all alone; the position of the lips is also a consideration. Compare “See” and “Sue.” Both have High vowels; “See” has a Front vowel and “Sue” a back vowel. What else differentiates them? It’s the lip-rounding; for “See” you pull the corners of your mouth back tightly while for “Sue” you round them into a nice little “O”