President Franklin Roosevelt was the first to name a personal envoy to the Vatican in 1939.He paid for the lodging out of his pocket as he was wealthy (helped with the votes I'm sure).
The Vatican claimed immunity under a U.S. law, the Foreign Sovereign Immunities Act of 1976, that allows foreign states to avoid being sued in court.
1984 Pres. Reagan raised the US envoy to the Vatican to full ambassadorship: " United States will have full diplomatic relations with the Holy See for the first time since 1867."
In 1867: "On hearing a rumour that the Pope was going to forbid American Protestants to hold services within Rome's city walls - services which were already being clandestinely held in private homes - the US Congress passed a resolution in 1867 saying that no federal funds could be used to maintain a diplomatic mission in Rome."https://www.theguardian.com/theguardian/2013/jan/11/vatican-usa-diplomatic-relations-1984-archive
A 1994 lawsuit against Pope John Paul II (search), also filed in Texas, was dismissed after the U.S. government filed a similar motion.
The U.S. Justice Department has told a Texas court that a lawsuit accusing Pope Benedict XVI (search) of conspiring to cover up the sexual molestation of three boys by a seminarian should be dismissed because the pontiff enjoys immunity as head of state of the Holy See.http://www.foxnews.com/story/2005/09/20/us-says-pope-immune-from-molestation-lawsuit.html
In the Oregon case, the Obama administration backed the Vatican and said the appeals court erred in ruling that a victim’s claim of sexual abuse by a priest falls within the exception to foreign sovereign immunity law. https://www.reuters.com/article/us-pope-abuse-usa/supreme-court-rejects-vatican-appeal-in-sex-abuse-case-idUSTRE65R3UB20100628
Trump appoints and the Vatican accepts an adulteress as Ambassador:
Trump's choice of Callista Gingrich to serve as U.S. Ambassador to the Holy See will require Senate confirmation. There is little sign of opposition to her nomination, but it is not without controversy. She carried on an affair with Newt Gingrich for several years while he was still married to another woman, and their relationship troubled many conservative Christians.
Could this be the reason why the US protects the Vatican?
"What we do know is that Vatican Bank, officially titled the Institute for the Works of Religion, manages €5.9bn ($7.3bn, £4.64bn) of assets on behalf of its 17,400 customers. And it manages €700m of equity which it owns. Another titbit to emerge is that it keeps gold reserves worth over $20m with the US Federal Reserve."
"The Catholic church, once all her assets have been put together, is the most formidable stockbroker in the world. The Vatican, independently of each successive pope, has been increasingly orientated towards the U.S. The Wall Street Journal said that the Vatican's financial deals in the U.S. alone were so big that very often it sold or bought gold in lots of a million or more dollars at one time."