I'm seeing three different arguments against the vaccine. Perhaps it's best to call them out separately.
1) CONTINUING sin. As the one article cited by Sean points out, we're not talking about a remote material participation in some past sin, but an active participation in a current sin. To the extent that remnants of the murdered child remain in the vaccine, we are actively participating in the abuse and desecration of a human body. Let's say that the nαzιs killed some Jews and made lampshades from their skin (assume for now that this actually happened). I didn't agree with what they did to the Jews, but I decided to acquire one of these lampshades and have one of them on my coffee table. Whether or not I agreed with or participated in the murder itself, I am participating in an ongoing sin against the dignity of that person. Likewise, going back to my car lot example. I want a cheap car so I buy one off a lot where I know that all the cars are stolen. Well, regardless of whether or not I participated in the original theft, by owning this car that I have not right to own in justice, I am continuing to actively participate in the injustice committed against the original victim.
2) WILLING the effect = WILLING its cause. Going back to the car lot example, I really want a cheap car, so I go buy one off that lot with the stolen cars. Now, I don't "agree" with the original theft and didn't participate in it ... so it's OK for me to buy one. "Shame on you, crook, for stealing the car, tsk tsk, but darn if I didn't get a cheap car. Thanks, guys; your sin was my gain." This is, in effect, what the vax-condoners are arguing, and, pardon my French, but it's total bullshit. In willing the effect of the crime (the cheap car), you're also implicitly willing its cause (the theft that caused your cheap car). So, back to vaccines, I want/need to get this vaccine, but I don't agree with the fact that it was made by means of (at least with the use of) a criminal desecration of a human body ... this is the same argument. It's a type of cognitive dissonance to claim you want the effect but don't want the cause ... a dissonance the causes a secretly dirty conscience. It's a supreme dishonesty, and God will call you out for it even if you manage to fool yourself.
3) grave sin of omission. If all Catholics refused to take this vaccine, this would hurt the profitability of the vaccine makers, and some/many/most of them might be deterred from using fetal cells because they don't want to eliminate a large part of the market for their product. They'll find some other way to get the vaccine created that would not immediately rule out a huge market share for it. So failing to refuse this vaccine is a grave sin of omission in deterring them from this evil.
When you combine all three of these, there's no way in which it can be considered remotely acceptable for a Catholic to receive such a vaccine.