Rabbis determined at the time after the holocaust that the medications Nazi’s created despite it being a medication, one cannot make use of a person who was killed and therefore using this medication is forbidden and is considered desecration of the dead. So currently vaccines, such as the chickenpox, is currently manufactured using "WI-38" human diploid cells. These human cells were retrieved from a 3 month gestation female fetus. My question is...Is this kosher?

The halacha is clear that saving a life supersedes all mitzvot, both positive and negative (aside from the 3 cardinal sins). Accordingly, it's just like it’s not only allowed but actually a big mitzvah and kindness to donate and have one’s organs used after one’s death (after rabbinical verification of halachic death), to save another person’s life (or even their sight!), which overtakes the mitzvah of honoring the dead body, so too regarding your question. In general, the Living Torah of the “King Who wants life” (Melech chafetz baChayim) places the needs of the living above those of the dead. How much more so it’s allowed and a mitzvah, to benefit from fetuses, who aren’t totally considered living beings, and even more so when the vast majority are not Jewish, as is the case in all the countries which manufacture the vaccines (we logically honor our Jewish brothers more than strangers). I’m not familiar with the details regarding the question of those murdered by the Germans.