Here's why:
* Minimal pairs are words that differ in only one sound, and this sound difference creates a difference in meaning.
* Vowel pairs are minimal pairs where the only difference is the vowel sound.
In this case, "doll" and "roll" differ only in the vowel sound:
* Doll: /dɑl/ (the vowel sound is similar to "father")
* Roll: /roʊl/ (the vowel sound is similar to "go")
The difference in vowel sound is enough to change the meaning of the words.