I did some testings with this too. I moved in a vampire with my mixologist, buy them a lots of plasma fruits from grocery store and other from buydebug (I couldn't find plasma fruit in buydebug).
With no nectar making skill Sim made famous 6 life fruit, 3 flame fruit,1 cherimola blan nectar, and it was amazing (excellent quality, 99$). Both Sim and vampire got sugar rush and astonishing aroma, but vampire didn't get cozy fire. They seem to be immune to flame fruit.
Other test nectar was 6 life fruit, 3 flame fruit, 1 plasma fruit, also amazing. It is excellent quality, 91$ bottle at normal quality ingredients and no nectar making skill, basic production, everything is the lowest possible

Vampire enjoy it as regular nectar, having astonishing aroma and sugar rush, while Sim got only sugar rush and nausea moodlet. This time none got cozy fire. Vampire's thirst was not improved.
Third one was 10 plasma fruit nectar, and same result. Sim got sugar rush and nausea, while vampire got usual (fabulous finish) and sugar rush.
I'm not sure how plasma fruit is usable except for vampires to eat it. Mixologyst ignore it in bar, and keep making drinks out of anything else, and if only plasma fruit (or garlic) is in bar, they use regular ingredients. Plasma nectar make Sim sick, and vampires can drink regular nectar and enjoy almost all of it's moodlets, and nothing special comes out of plasma nectar. It is outstanding fertilizer, but my gardener choose life fruit over plasma fruit, so I guess it is not the best one.
I even tried to train vampire in mixology to use plasma in drinks, but he kept using regular ingredients beside both plasma and other harvestable such as grapes, even with maxed mixology.