I added the sweetener just before serving, because I tasted the liquor, and judged it to be overly tart. My hunch is that if the sweetener were added early, there could be considerable variation in the tartness of the fruit (a function of the ripeness of the individual plums when they are cut open), so the final product might occasionally be cloyingly sweet. "You can't fix cloyingly sweet."
I used a vodka imported from France by Sazerac Company (owned by Bill Goldring), with EPIC(R) on the label. It is labeled as a product of Canada, so I assume it was distilled in France, exported in a bulk container to Canada (up the St Lawrence Seaway... to the port of Brockville maybe?), then bottled and labeled for the US market.
I think it was 9 days before I brought the liquor jar out for the first time.
The linked website is a real-time web cam view of the Brockville ON waterfront. http://brockville.com/brockville-live-cam