Fellowship of the Dice

Dug something up for you, doing my best to avoid spoiling myself on either campaigns. No, Skulls and Shackles begins at or later than 4681.

https://pathfinderwiki.com/wiki/Free_Captains

The Free Captains have protected rebellious Sargava against House Thrune and the Chelish Navy since 4640 AR, when they started taking annual tribute from Baron Grallus. This includes victories by the Free Captains in 4643 AR and 4660 AR.[5] While most Free Captains abide by the treaty, the dissenters of Firegrass Isle refused to obey it, creating a rift between them and the Pirate Council.[6]

The Free Captains united under the Hurricane King in 4674 AR to harass shipping north of the Shackles, and the first Free Captains' Regatta was held three years later. Their recent success spurred the so-called Glorious Age of the Free Captains that began in 4681 AR and represents the unprecedented era of prosperity that the Free Captains currently enjoy.[7]

4638: Riddleport founded, by Sir Aurek Roderic.
4640: Free Captains start taking tribute from Baron Grallus.
4674: Kerdak Bonefist becomes the leader of the Free Captains.
4680: Gaston Cromarcky becomes the current ruler of Riddleport
4681: The beginning of the Free Captains' golden age of piracy. (Skulls and Shackles campaign takes place in this era)

4718: Roughly the beginning of Return of the Runelords (date set by ManWithDoor)

The wiki I use doesn't say when the Shackles' age of piracy ends. So it depends on Vardaen, because I haven't found any hard dates in his campaign or in the wiki that would help me with that. It looks like it is purely up to him whether he has his campaign set during the Return of the Runelords. It could very well be after.