Vowels are difficult to describe in that, unlike consonants, it’s impossible to give an exact position of the speech organs.
For example, the bilabial plosive /p/ is produced by first bringing the lips together (bilabial) to form a complete closure, air pressure is built up behind the lips and then released (plosive). But, if there was a space between the lips (so that air could pass through) during the production of the sound, we would be talking about a fricative and not a plosive anymore.
The point is that a consonant can be a stop or a fricative but not somewhere in between. This is possible, however, when describing vowels.
For example, we can say that to produce /uː/ you arch the back of your tongue toward the soft palate, close but not touching. But for /ʊ/ you do the same, except that the tongue is not as close to the palate as it is for /uː/.
That is to say, the boundaries between vowels are not absolute; rather, they’re on a continuum.