Different jobs, different tools.
Mobile Passport Control is a free US Customs app that speeds up entry into the United States for eligible travelers. It does not get you a visa. It shortens the line after you land.
eVisa portals are government websites where you apply for permission before you fly: Vietnam's evisa.gov.vn, Turkey's evisa.gov.tr, India's indianvisaonline.gov.in. They decide whether you board.
You need the eVisa portal when your destination requires advance permission. You need Mobile Passport Control when you are entering the US and want a faster lane. Frequent travelers to both need both.
Sources: U.S. State Department, IATA Travel Centre
Confusing the two is common and costly. Downloading MPC the night before a Vietnam trip helps exactly nothing. Match the tool to the border you are about to cross.