Question

Is the only difference between alliteration and consonance the placement of the vowel sound, or is there something more?

Answer 1

In contrast to alliteration, consonance involves repetition of consonant sounds only .
https://literarydevices.net/consonance/

Explanation:

Alliteration is usually a longer string of words. Consonance may occur within a word or short phrase. Alliteration is a stylistic device in which a number of words, having the same first consonant sound, occur close together in a series.

An important point to remember here is that alliteration does not depend on letters but on sounds.

Examples:
But a better butter makes a batter better.
A big bully beats a baby boy.
Many more here:
https://literarydevices.net/alliteration/

Consonance refers to repetitive sounds produced by consonants within a sentence or phrase. This repetition often takes place in quick succession such as in pitter, patter.
Many good examples:
https://literarydevices.net/consonance/