Quick search:
http://www.processing.org/discourse/beta/num_1276207292.html
https://forum.processing.org/topic/how-to-export-sketches-with-transparency-for-high-resolution-printing
https://forum.processing.org/topic/transparent-image-in-pdf-file
http://processing.org/discourse/beta/num_1252603890.html
and so on...
No ready-made solution, alas, just that you are not the first to have this issue...
"I still need scalable vector graphics"
Maybe you can try SVG instead?
http://www.processing.org/discourse/beta/num_1276207292.html
https://forum.processing.org/topic/how-to-export-sketches-with-transparency-for-high-resolution-printing
https://forum.processing.org/topic/transparent-image-in-pdf-file
http://processing.org/discourse/beta/num_1252603890.html
and so on...
No ready-made solution, alas, just that you are not the first to have this issue...
"I still need scalable vector graphics"
Maybe you can try SVG instead?