What is the best way to get from the ship pier at Vodice to the Palace in Split.
That's a fairly long distance, around 100km. You might need to arrange a private transfer, especially if Vodice is only a cruise stop.
We took the bus from Sibenik to Split, I think it was a little over an hour. It made a couple of stops and one of them could have been Vodice but I honestly don't remember. Check buses and see if the bus stops there. If it does then it's a rather easy bus ride. Diocletian Palace is a 5-10 minute walk from the bus terminal in Split.
You can use this link to determine whether bus schedules would fit your day trip itinerary: http://www.autobusni-kolodvor.com/en/default.aspx. The Vodice bus station is near the marina and the Split bus station is near the palace. "Best" may differ depending on whether you must to be back at the Vodice ship pier for a set departure time or you have a more flexible schedule (e.g. you're staying that night at lodgings near the pier).