When you say "America," I assume you are speaking about the United States. Before the Panama Canal, a ship going from Vietnam to South Carolina by the shortest route would have to cover 20,000 miles/32,000 km (I used the Measure Distance feature in Google Maps). If the went directly to Panama and hiked across the isthmus (during which most of your cargo would have died or escaped) from the Pacific side to the Atlantic side, it cuts the distance to 16,000 miles. Keep in mind the horrible mortality rate of slave ships. Businessmen who cared more about profits than human life would reject this scheme this as impractical.

The route from the slave-trading ports on the west coast of Africa to South Carolina was a little over 6,000 miles/10,000 km. This is why the 338,000 slaves brought to the United States came directly from West Africa. This constituted about .03% of the Atlantic slave trade.

However, most of the slaves (10.4 million) imported from Arica to the new world went to the Spanish, French and Portuguese colonies along the Atlantic side (Brazil, the Caribbean). It's 5,400 mile/8,700 km from Whydah to Cuba, 2,000 mile/3,000 km to Brazil.

Having said that, there's an interesting video called Coolies: How Britain Reinvented Slavery about the system of indentured immigration that sent a million people from India to locations all over the world, most never to return to their homeland. This is why Indians are one of the most dispersed ethnic groups in the world, with large populations in East Africa, Caribbean and Pacific islands, and other places. The British public had demanded the end of slavery in the 1830's, and businessmen had to find a way to continue their profits.