Vaccines and plasma serve 2 different purposes. A vaccine will provide long term protection, but do nothing to help a patient who is sick now [0]. In contrast, plasma will help a patient who is infected now, but provide no long term protection.

[0] Or, at least, the Covid vaccine has not been proven to do anything to help a patient who is sick now. There are cases (such as rabies) where getting vaccinated after being infected is still beneficial.

At any point there are millions or tens of millions of people with COVID. Can’t really make much of a dent with plasma.