Did the foreign policy actions of the United States reflect selfish, imperialist ambitions or did the USA act as a concerned "big-brother" who was looking after the interests of the western hemisphere?

The correct answer to this open question is the following.

Although you did not specify the time in history or any specific context, we are going to assume that you are referring to the US imperialistic role through history.

Being that the case, we can comment on the following.

The foreign policy actions of the United States reflect selfish, imperialist ambitions since the times of President James Polk and the Manifested Destiny. He expanded the US territory acquiring the Mexican northern states after the Mexican-American War.

Since those years, passing through the times of President Monroe and the Monroe Doctrine and President Theodore Roosevelt and his Roosevelt corollary, the United States has always had interventionist purposes as part of its foreign agenda.

Nobody has granted the US the right or role to be the "big brother" who was looking after the interests of the western hemisphere. That is why the US has had many problems throughout history in the Middle East, Latin America, Europe, and other regions.

during the 20th century, the role of the vice president has evolved into more of an executive branch position. now, the vice president is usually seen as an integral part of a president's administration and presides over the senate only on ceremonial occasions or when a tie-breaking vote may be needed.