“the use of such vaccines must never be advanced as mandatory, or as a universal duty. Because some of us in conscience believe that we are called to refuse to take them.” Source
In other words, the Magisterium cannot require us to follow Church teaching on the moral law because “some of us” have decided to disregard that teaching and follow our own consciences, in contradiction to the Magisterium.
The teaching of the Church on cooperation with evil is very clear. A vaccine which has a remote connection to a past abortion, is not intrinsically evil. And there are good reasons, good intended ends, for taking the vaccine. Therefore, the morality is based on an evaluation of the circumstances. During a pandemic, a vaccine against a deadly disease has weighty good consequences. The bad consequences of accepting a vaccine with very remote material cooperation to a past abortion are very light. The most weighty bad consequences are unrelated to the past abortion, such as the risk of harmful side effects. You can refuse to take the vaccine, if you think that the harmful side effects outweighs, morally, any good consequences. But there is nothing of sufficient weight in the remote material cooperation to justify its refusal. Refuse the vaccine if it will do more harm than good. But its relation to a past abortion is not of sufficient moral weight by itself to justify rejecting a vaccine against a deadly disease (which can also turn into the chronic illness LongCovid) during a pandemic.
The reason that people are crying out against this vaccine is that it undermines their precious political agenda, which has replaced the moral law and the teaching of the Church for many Catholics. That’s right. Many Catholics have politicized religion so much that they are motivated even in grave moral matters solely by the effect on their political position, rather than the good of the human race. And this is particularly clearly seen in conservative Catholics who, against all reason and science, claim that the pandemic is not severe, is no more dangerous than the flu, or is a hoax. And included in these pandemic-deniers is Bishop Vigano, one of the most revered leaders of the far right in Catholicism, one of the most outspoken papal accusers.
They do not reject the vaccine because of an evaluation of the act under Catholic moral teaching. The connection to the past abortion is too remote for that evaluation to make sense. And they don’t reject it based on an ordinary evaluation of present-day consequences (apart from cooperation). They reject it because accepting a vaccine related to an abortion weakens their political stance against abortion.
“the use of such vaccines must never be advanced as mandatory, or as a universal duty. Because some of us in conscience believe that we are called to refuse to take them.”
This is part of the rejection of the Magisterium that is occurring today. It is as if they are saying: “The Magisterium can’t make us believe what it teaches because we have decided to form our consciences based on what the conservative Catholic subculture teaches instead. We must never be compelled by the Church to believe what the Church teaches, because we have created our own Magisterium separate from the Pope and Bishops.”
Should you take the vaccine? Weigh the possibility of severe side effects, esp. if you are elderly or have chronic conditions, and weight the benefits of good but still limited protection against the pandemic. But ignore the very remote relation to a past abortion. It carries too little weight in this particular case, due to the severe dangers from this disease, and the very remote connection to the past abortion.
RLCJ
the reproach of Christ 
You go into the doctors office and two vaccines are placed in front of you. One with aborted fetal cell lines and the other without aborted fetal cell lines. You ask the doctor and he says both are equally effective. You know in your heart from research and the doctor tells you which vaccine has the aborted fetal cell line. You choose the aborted fetal cell line one. Have you sinned?
Would the circumstances of the use of more and more vaccines with aborted fetal cell lines lead to more abortion?
No vaccine contains aborted fetal cells, fetal cells of any kind, nor cells from a line of cells that began with an abortion. Vaccines contain what vaccines usually contain, either RNA or DNA and a vehicle for delivering that to the body; or, alternately, a protein from the virus, or a whole virus that has been “killed”. No vaccines contain fetal cells.
All vaccines for Covid-19 are not equally effective, and each vaccine has serious risks, different for each one. So it would be sinful to disregard the weighty moral factors in the circumstances of this choice, and instead decide solely based on very remote material cooperation. The Church teaches we must weigh all the relevant circumstances and consequences. My evaluation of that circumstance is that it carries very little moral weight, and that the effectiveness, benefits, and risks of the vaccines are what should determine the moral decision. I have weighed the use of the cell lines and found that whether the vaccines was produced or merely tested using fetal cells, there is only a slight difference to the moral weight, which is far outweighed by the present-day considerations of benefits and detriments of the vaccines.
I would choose the vaccine that is safest, over the one that is more effective. But that is actually not clear.
Also, I know of no places that give you a choice of vaccine.
Would the circumstances of the use of more and more vaccines with aborted fetal cell lines lead to more abortion?
My understanding is that the cell lines are from an abortion decades ago, which was not done to collect the cell lines. No new abortions would have anything to do with vaccines.
I am not getting a choice, but if I were the degree of connection with that abortion would be a secondary factor for me. If both vaccines were equally safe and effective then that would decide me.
If you wish to take that factor into account, given equal vaccines in other aspects of circumstances, you can. But I don’t believe that is the case. They are not equally safe or equally effective. And it is wrong to ignore more weighty moral factors, in favor of far less weighty moral factors that are being over-emphasized by certain leaders in the Church because of the political weight, not the moral weight, of the decision.