Why do Catholics believe that Jehovah is God?

[aquinas138]

They are indeed the same vowels as Adonai, except that the vowel takes a different form under yod (the vowel scheva in *yehova) as opposed to under aleph (the vowel hataf patah in 'adonai); this is just a phonological rule in biblical Hebrew. Anyway, this is the reason the vowels look different at a glance; nevertheless, the two vowels are allophones.

So this wouldn't affect the pronunciation? It's just different graphically?

Well, technically it affects pronunciation, but subphonemically. If you're not familiar with linguistic terms, that just means they are different sounds from the standpoint of the mechanics of how the vocal apparatus produces sounds, but not from the standpoint of meaning. Allophones are two different sounds that are, within a particular language, not considered to be different for purposes of meaning (this is a very non-technical definition).

For example, if you pay close attention, the "t" in the words stop and water are different sounds. In English, these are allophones; if you try to over-emphasize and use the "hard t" found in stop when pronouncing water, other English-speakers would think it sounds a bit odd, but would not think you were saying a different word. In Spanish, however, these two sounds are not allophones.

That's what's going on with the scheva and hataf patah in the Tetragrammaton and "Adonai"; the two vowels are allophones in Hebrew, and hataf patah is required under glottal consonants in Hebrew phonology.