The vice-president is second-in-command, and will act as president if the president dies, resigns, or is removed from office.
My initial thought, while I was reading this, was there was a mistake and we needed to put the between as and president:
... and will act as the president if ...
The vice-president will become the president, the only one in the country.
Why are we omitting the here? Any help would be appreciated.
"The vice-president" and the second occurrence of "president" is referring to a person. Like you said there was only one president or vice-president in a country. In this case, the definite article should be used.
On the other hand, "will act as president" is referring to the presidential institution. Zero article is used when referring to some institution.
Look at the example taken from here
He was taken to court to be tried; in the court he met an old friend.
As you can see, the definite article is used when a speaker refers to a building. Reference to an institution has no article.
