Peace, the final attack on Saigon happened in April, 1975. By then, the only US troops left in Vietnam were the embassy security guards. Two of them were killed in the process of getting the civilians out.

Blackwater and its ilk are used primarily as security guards, "protecting" high-profile civilians and non-military convoys. They rarely, if ever, participate in actual military operations - that is left for the Army's $24,000/year soldiers, $35,000/year sergeants and $48,000/year lieutenants. Many of the contractor employees, BTW, are from third-world countries and don't get the rumored six-figure salaries.