should therefore, as I have always maintained (Gawd, it gets lonely up here!) be a vowel

The letters I and V were used by the Romans to represent both straight vowels and semi-consonants. Examples :
In VINVS (vinus) the I is a straight vowel; the first V is a semi-consonant, similar to our W in sound. The second V is a straight vowel. In IVLIVS (Julius) the first I is a semi-consonant similar to our Y sound and the second I is a straight vowel. In MAIESTATIS the first I was a semi-consonant, a glide between the A and the E. Over the years the pronunciation of the glide took on voicing and eventually a plosive beginning and became the J of our modern majesty.