God left us His name in the form of יהוה which is unpronounceable.
So, we need to add some vowels יְהֹוָה to pronounce it “Jehovah” in English
The question is: what vowels are they and who added them?
The answer is: they are the vowels of the Hebrew word “Adonai”
First off, you need to get the, 'the vowels came from 'adonai' out of your head. As can be seen that there is the vowel E in Jehovah and there is no E in 'adonai'.
You rightful note that God left us his name in the form יהוה
Here are 4 more names that are consonants only and need vowels added to pronounce:
יוסף יהודה ישעיה יהושע
Those names are:
YeHOshua, YshaYaH, YeHUdaH, YOsef
Notice that the vowels E, O, A, can be found in them. Keep in mind that these names never stopped being written or pronounced. These names have in them the same consonants as the name God left us in regards to his name. The consonants are Y H V. יהו
When you look at the names written in Hebrew you can see these consonants. (YHV) When you look at the names written in English you see the YH but no V. That is because the V is a holding marker for the vowels U and O.
In 3 of the names above the V is in them but 2 are replaced using the O and 1 is replaced using the U. So based on those names you have the vowels E and A and the vowels U and O because of the V.
So here are the vowels we are dealing with to insert in the consonants of God's name: E, O/U, A
Keep in mind that it will be either a O or U, not both. And the O or U replaces the V, they are not added in addition too.
It is YeHOshua, not YeHOVshua
It is YeHUdaH, not YeHUVdah
Likewise, in using those vowels with the consonants of God's name it would be:
YeHOaH, not YeHOVaH
Or in today's English because we use the J instead of the Y it would be:
JeHOaH, not JeHOVaH
The main take away is that putting aside the mistake of keeping the V in, the appropriate vowels to use for God's name are E, O, A. This is based on the comparison of other Hebrew names (that never stopped being spoken or written) that contain the prefix and suffix of God's name.
JeHOshaphat – JeremiaH
JeHOiakim – IsaiaH
JeHOram – NehemiaH
JeHOiachin – ZechariaH
יהו (YeHO) + ה (aH) = YeHOaH/JeHOaH
BTW, the names above YeHOshua, YshaYaH, YeHUdaH, YOsef
Are known today has: JOshua, IsaiaH, JUdaH, JOseph