"Fr." Zuhlsdorf was "ordained" in 1991, hence in the dubious Novus Ordo rite.
It shows in his snarky unpriestly demeanor.

In the autobiography of Father John Gerard, “The Hunted Priest”, there is an account of Father Southwell’s defense of equivocation. The book is a great read, but the pertinent part starts at page 260:


https://archive.org/details/adg3675.0001.001.umich.edu/page/259/mode/2up

It was explained to me as "mental reservation". 

For example:
- you take a child with a broken arm to the ER
- ER asks is the child is vaccinated
- if the child is, say so and no problem
- if the child isn't, no matter the reason, the parent will be grilled, treated as a criminal, and, possibly have their child taken away.  In this instance, I was told, it would be o.k. to say the child had been vaccinated.

I was  also taught that a lie must involve deceit.
For instance, telling someone the trees are purple when they know the trees aren't purple, is not a lie because no deceit is involved.

In the example you give, that is not a mental reservation, unless you were to mean "yes, he has been vaccinated in the past, but for different things".  The broader mental reservation of "yes, he has been, as far as you are concerned", is something I have heard of, but I'm not comfortable with it.

In the example you give, that is not a mental reservation, unless you were to mean "yes, he has been vaccinated in the past, but for different things".  The broader mental reservation of "yes, he has been, as far as you are concerned", is something I have heard of, but I'm not comfortable with it.

I’m fairly comfortable with that mental reservation, but what I wouldn’t be comfortable with is if there were any follow up questions.

Are there any traditional Catholic experts in moral theology?  It would seem to me this is an area that divides traditionalists to them point of sinning against charity when we disagree with one another. 

I wouldn't call myself an "expert", but when I first entered the Catholic world, the better part of five decades ago, one of the first things that captivated me, was a clear moral code, no exceptions, absolute right and wrong, and, to be perfectly honest, I finally had an answer for why the world, and people, are as ****ed up as they are.  When it's all up to individual conscience and "personal moral codes", then you have chaos, and people always find a way to do what they want to do, especially where money, pride, and sex are concerned.  Find me a divorce where you didn't have at least one of those three.  Spousal abuse is about the only other thing that comes to mind.

So where am I going with this?  Yes, there is right and wrong, and (to borrow the Mormon phrase) you "choose the right" regardless.  But there are some things on which moral theologians differ, more in degree than in kind (but sometimes that too), and the Church doesn't offer one, black-and-white, definitive answer to it.  Where to draw the line between mental reservation and lying is one of those.  Some authors would even say that outright lying --- not just mental reservation --- is permitted in extreme cases.  I have to think that, in some cases, in dubio libertas comes into play.  You have different opinions and different shades of gray.  The decision you make, whether X is a mental reservation, or whether Y is something so drastic that you may tell that lie, might have to come down to individual conscience --- a word I strongly dislike due to the way it is abused in the modern Church, but still, it is something that exists, Newman troubles me when he calls it the "aboriginal vicar of Christ" and says he would drink to conscience before he would drink to the Pope, but he may not be totally wrong.  

When you're in a drastic situation (nαzιs at the door hunting down the Jєωs you've got hidden in the attic), you can't do like in the movie "Click" and freeze the action, while you figure out what to do, or run and ask your confessor.

In the example you give, that is not a mental reservation, unless you were to mean "yes, he has been vaccinated in the past, but for different things".  The broader mental reservation of "yes, he has been, as far as you are concerned", is something I have heard of, but I'm not comfortable with it.

More like, "yes, he has received all the vaccinations he's going to get."

So, I guess it is not mental reservation, so much as knowing what they're really asking but not answering with what they want.

The first thing which came to my mind when I read the thread title was, “Didn’t Pope Pius XII forge (or supply Jєωs with) fake birth certificates?

If so, was it human weakness (sin), or permitted for some reason?

I thought that was the future Papa Roncalli and perhaps orders who interpreted Papal orders in their own way. Pius did request Orders to shelter those at risk of persecution.