[2] Non-ratifying parties, such as the U.S, have recognized parts of the VCLT as a restatement of customary international law.

[5] During the 20 years of preparation, several draft versions of the convention and commentaries were prepared by special rapporteurs of the ILC, which included prominent international law scholars James Brierly, Hersch Lauterpacht, Gerald Fitzmaurice, and Humphrey Waldock.

[8] Over two sessions in 1968 and 1969, the Vienna Conference completed the convention, which was adopted on 22 May 1969 and opened for signature on the following day.

Article 11 defines "means of expressing consent to be bound by a treaty" including ratification, acceptance, approval or accession.

[15] However, sometimes a specific set of non-member states or non-state actors may be invited to join negotiations.

If the treaty is restricted to Members of the United Nations or Parties to the Statute of the International Court of Justice, there is no ambiguity.

However, a difficulty has occurred as to possible participation in treaties when entities that appeared otherwise to be States could not be admitted to the United Nations or become Parties to the Statute of the International Court of Justice because of the opposition for political reasons of a permanent member of the Security Council or have not applied for ICJ or UN membership.

[19] Some treaties that use it include provisions that in addition to these States any other State invited by a specified authority or organization (commonly the United Nations General Assembly[citation needed] or an institution created by the treaty in question) can also participate, thus making the scope of potential signatories even broader.

The present Convention shall be open for signature by all States Members of the United Nations or of any of the specialized agencies or the International Atomic Energy Agency or parties to the Statute of the International Court of Justice, and by any other State invited by the General Assembly of the United Nations to become a party to the Convention, as follows: until 30 November 1969, at the Federal Ministry for Foreign Affairs of the Republic of Austria, and subsequently, until 30 April 1970, at United Nations Headquarters, New York.Articles 31–33 of the VCLT entail principles for interpreting conventions, treaties, etc.