"Without a doubt, the agreement signed in Paris to end the war and restore the peace in Vietnam was a critical turning point in the beginning of the end for South Vietnam. Although the Nixon Administration fully intended that South Vietnam would remain free and independent, the fact remains that the agreement was only successful in withdrawing U.S. troops from South Vietnam and obtaining the return of American POWs from North Vietnam. Hanoi had good reason for strictly complying with these two terms: by removing the U.S. from the war, North Vietnam was free to rebuild after the devastation it suffered during the 1972 "Easter" offensive and prepare for its final offensive in 1975 to conquer all of South Vietnam"
...
"If these officers could have succeeded in making significant reforms within the armed forces, then the RVNAF may have been able to thwart the NVA offensive and quite possibly made a difference in the final outcome of the Vietnam War. However, events proved that the deficiencies inherent in the Thieu regime eroded the support of its people and soldiers; thus, the collapse was inevitable. Also, the political climate in the United States during this time period would not allow for U.S. support to the government of South Vietnam to continue indefinitely. Although these younger officers could have eventually fostered the necessary reforms, the South Vietnamese needed to make quick changes and simply ran out of time in 1975."

Here is some stuff:
http://www.ehistory.com/vietnam/essays/theend/0028.cfm