I know that the changes made by Paul 6th allow water at any time, but what about the rules under Pius 12th? Do those rules allow for water at any time or just 1 hour before? And what about medicines? And would a supplement like Vitamin D break the fast?

From Tradition the fast was very strict, no food or water allowed from midnight the night before, not even a drop of water. This would of course, exclude some water swallowed in accidentally if one was cleaning their teeth.

Pius XII changed this to 3 hours for solid foods and 1 hour for non-alcoholic liquids.

and,

"The infirm, even if not bedridden, may take nonalcoholic liquids and that which is really and properly medicine, either in liquid or solid form, before Mass or Holy Communion without any time limit."

Vitamin D (unless it is really and properly a medicine - doctor prescribed) would not apply.

1957
https://www.papalencyclicals.net/pius12/p12fast.htm

The main thing to remember, is that all this lessening of the Eucharistic Fast was supposedly because of the clamor of bishops who wanted to have more afternoon/evening Masses.

Pius XII still exhorted the faithful who were able to observe the midnight fast:

1953
https://www.papalencyclicals.net/pius12/p12chdom.htm

Within he stated,

"The law of the Eucharistic fast from midnight continues in force for all of those who do not come under the special conditions which We are going to set forth in this Apostolic Letter. In the future it shall be a general and common principle for all, both priests and faithful, that natural water does not break the Eucharistic fast."

The real thing one should ask themselves is why did the Church preserve the midnight fast from food and drink for so many centuries? The answer is of course to foster greater devotion to the Blessed Sacrament. Christ is the sustenance of soul and body, He is the "Bread that came down from heaven". He is the "fountain of living water", etc. Mortifying the flesh through fasting draws the mind to heavenly things and makes us more fit to participate in the heavenly mysteries.

IMO, we must do the minimum (3 hr. food/1hr. drink), and we are "good", but, if possible, we should do the Traditional thing, and we are doing "better" by observing the midnight fast as called for by Pius XII.  In the end, it is us that wins out, because we are the poor beggars who need grace and God sees with what spirit we approach Him at the Altar and main thing is to have humility of heart - fasting helps foster this spirit if it done with that intention. Though it does take some getting use to.

What if you are in the choir and need a drink? I thought there was a second decree where Piis 12th changed the rules again

It is there in the body of the text I already cited,

"The law of the Eucharistic fast from midnight continues in force for all of those who do not come under the special conditions which We are going to set forth in this Apostolic Letter. In the future it shall be a general and common principle for all, both priests and faithful, that natural water does not break the Eucharistic fast."

God bless you

We somtimes visit England where the mass isn't until 12:00 midday. Exceptionally we do have a breakfast on these occasions

I know we are not supposed to "wing" it and make up our own rules, but I do the overnight fast for food, but I do the three hour fast for coffee ( with milk or cream). Our Mass is at 12noon on Sundays and we travel and hour to get there.

Before the rule change, my mother rarely if ever went to Communion because she was underweight and somewhat sickly due to poverty and insufficient medical care.  She’d get dehydrated and pass out.  In those days, most Catholics did not go to Communion every Sunday.  Priests didn’t hand out dispensations easily. More than once, she passed out during Mass to the point where she was made to sit in the children’s pews on the outer aisle in back. (Children all went to 9:00 Mass and sat together, supervised by the sisters.)