Preheat oven to 400°F.
Trim the pork tenderloin of any excess fat. Use a mortar and pestle to crush the peppercorns, marjoram, rosemary and garlic powder together. Sprinkle the spices around the meat and wrap in plastic wrap. Set aside for up to 15 minutes. (If you are going to let this rest longer, place in fridge and remove 15 minutes before cooking.).
To roast plums, place the fruit skin side to the bottom of the pan and leaves from 2 rosemary sprigs in an 8-inch-square baking dish.
Whisk water, balsamic vinegar, 4 tablespoons sugar and peppercorns in a small bowl until the sugar dissolves. Scrape seeds from vanilla bean; add the seeds and bean to the vinegar mixture. Pour the mixture over the plums. Sprinkle with the remaining 2 tablespoons (turbinado or white) sugar.
Roast the plums, uncovered, until tender and beginning to break down, 20 to 25 minutes.
Once the meat has sat for 15 minutes, heat olive oil in a cast iron skillet (or another oven proof pan). Cook the meat for 5-6 minutes turning so that all sides of the tenderloin are browned. Place skillet in the oven and cook until the meat reaches 155 degrees, about 12-15 minutes. Once done, remove the pork from the oven and let rest for 10 minutes. (In that time the meat will reach the desired 160 degrees.).
Once the plums are done roasting, discard the rosemary and the vanilla bean. Strain the roasting liquid into a small saucepan and bring to a boil. Reduce heat to medium-high and cook until reduced to 1/2 cup, 6 to 8 minutes. Whilst reducing the sauce tent the plums with foil over the roasting pan. (I would suggest not starting your reduction until the meat is cooked and resting.).
After the meat has rested, carve on the diagonal into slices about 1/4 to 3/8 inch thick.
To serve place a quarter of plums onto the plate. Place the pork on top and drizzle with the balsamic reduction. Garnish with additional rosemary if desired.