Covid-19 convalescent plasma is still investigational; meanwhile blood donation organization are collecting it. They will not collect it if one has received a covid vaccination even if one was covid positive in the past.
I am not studied up on the subject, but even the blood donation workers found the restriction puzzling. I could only speculate to them is that using vaccinated blood plasma has not gone through any studies yet and therefore they do not want to mix it into the pure convalescent plasma supplies.
Updated information for health care providers regarding treatment of COVID-19 with convalescent plasma
www.fda.gov