A legal test of the constitutionality of sending a US envoy to the Vatican could be very complicated, say proponents as well as those who oppose such a move.
Those who oppose the move on church-state grounds, including the Baptist Joint Committee on Public Affairs and Americans United for Separation of Church and State (AU), admit that their first priority is to try to block the Senate's confirmation of President Reagan's nominee, William A. Wilson.
If unsuccessful, they will take the matter to the courts. However, this would be a ''last resort,'' says John Baker, constitutional lawyer and general council for the Baptist Committee. Dr. Baker says the appointment would be opposed on three long-established constitutional grounds:
* That it constitutes preferential treatment of one church over another.
* That it promotes an ''entanglement'' of the state in religious affairs.
* That it would tend to encourage divisiveness among religious groups.
However, AU official Joseph Conn - who also strongly opposes the Vatican appointment on church and state grounds - allows that a legal resolution of the issue would also raise questions about the President's authority in foreign-policy matters. Diplomatic issues have been the province of Congress and the President, not the courts, he explains. ''If the Reagan administration had decided to set up a relationship with the National Council of Churches, we would have no problem striking it down,'' Mr. Conn says.