Donnelly is resigning as Ambassador to the Vatican (can anyone explain why that is its own diplomatic posting instead of a dual role for the Ambassador to Italy? We have a dual Ambassador for New Zealand and Samoa).
The Vatican City is significantly more important than Samoa.
Some people would dispute that. Samoa is important as concerns Pacific naval security and right next to U.S. possession American Samoa (so obviously we don't want them to be friends with China).
What does the Ambassador to the Vatican do all day? What does the office of the Ambassador do all day? There's no visa processing to worry about.
I get it's a plum job and you get to live in Rome, it's questionable whether it should exist as concerns government efficiency.
The cost of paying for one ambassador is a drop in the bucket compared to the benefits of having a direct line and close relations with the Catholic Church. Meanwhile Samoa is entirely irrelevant to naval security as it has no suitable deepwater anchorages for modern warships. There's a reason America holds control of only eastern Samoa - Pago Pago Harbor is one of the largest and deepest natural ports in the world.