How many Americans had heard of Vietnam before Spring of 1964?

A large majority. The public didn't know about it or didn't care about it. Not even the politicians in the senate or the house of representatives knew about it. For those who did know, Vietnam was some random South East Asian country where communists wanted to control things so they didn't care to learn more about it, they only cared about fighting communism. After the war began, everyone found out about it.