Add the plums to a saucepan, along with the water and bring to a simmer over medium-high heat. Simmer for 15 minutes.
Strain through a fine sieve, pressing to extract juice, discard solids, then strain liquid through muslin or paper towel and pour the liquid into a measuring jug and note how much liquid remains.
Return to the saucepan to a medium heat and combine the plum liquid with an equal measure of sugar. Stir until sugar dissolves.
Add vanilla seeds and the lemon and orange juice, bring to the boil, reduce heat to low-medium and simmer for 5 minutes.
Remove from heat, add vinegar and stir to combine, then transfer to a sterile glass bottle, seal and refrigerate.
Notes
Serve as a mixer with lemonade or mineral water, or in cocktails with Vodka and Gin. It's also lovely added a glass of champagne.