Did Congress really cut off funds to South Vietnam?

In a word, no.

U.S. military assistance to South Vietnam:

Fiscal Year 1974 - $813 million.
Fiscal Year 1975 - $700 million.

Fiscal Year 1975 ran through September 30, 1975.

Saigon fell on April 30, 1975.

Congress only cut military funding to South Vietnam by 14% in its final year of existence, but, that cut combined with increased fighting against the North Vietnamese and a 12% inflation rate that year that made things like ammunition and spare parts more costly, ate up the money fast.

Here is the best write up I found on the subject.