Operation Frequent Wind was the final evacuation of Americans in Vietnam as well as, those Vietnamese deemed dangerous/hostile to the communist regime and as many Vietnamese people we could evacuate as well. The South Vietnam Forces put up a hastily defense before being overrun and North Vietnam tanks crashing through the palace gates.

Once North Vietnam took over Saigon and the major airport, there was no way for them to really evacuate since America had already left to go back to the Seventh Fleet and out of South China waters. From then on, the only way out was to be smuggled out by sea or any means. Now I cannot find the source but provided one account of smuggling out South Vietnamese citizens, here.

The USS Kirk was also involved in saving the last remaining vessels of the South Vietnamese Navy and saving thousands of refugees as well, here is the source.

Getting back to the main point of your question, were there any documented evacuations past Operation Frequent Wind? The answer to this, to the public with means of accessing non-confidential information, is no. Who knows though of remaining CIA, NSA, and various Special Operation missions that went back into Vietnam to rescue more people.

This also gives to why the Ford Administration enacted the Indochina Migration and Immigration Act, which allowed all refugees from Vietnam to be processed at Guam and eventually claim asylum and later citizenship into the United States.

I wish I could help you further but most likely the best way to go abotu finding more information on the subject is (if I had access I would have gladly researched into it more) to go into JSTOR or some other Primary Source archive site and try to find testimonials of refugees who were smuggled out by illegal means not by US/ARVN forces.

Source for Operation Frequent Wind and Operation New Arrival

Source for Indochina Refugee and Immigration Act 1975