Excerpts from Books and Wikipedia
"Both sides agreed that the South Vietnamese people had an absolute right of self determination in their system of government. Reunification of North and South would be addressed only through negotiation. However, in the agreement, the North Vietnamese forces stationed in the South were not mentioned. This meant that they could stay in place and pose a permanent threat to the stability and security of the South. Once it seemed certain that nothing would persuade the Americans to return to Southeast Asia, the North Vietnamese would feel free to brush aside their agreement and re-unify the nation by force. In April 1975, this is precisely what happened." [Furtado: 1001 Days]