This newspaper ad from Sunday, 2 June 1945 says:

At Confitería Congo. Today and every Sunday.
Rodolfo Biagi with singers Ortiz (Jorge Ortiz) and Amor (Alberto Amor)
Early show (“vermouth”) from 6p.m. to 10 p.m.
Late show from 10 p.m. to 3 a.m.
Avenida Forest 717.  54-6330 (telephone)

Vermouth, just like the name of the liquor, was the nickname given to the early show.  This is similar to saying “Happy Hour.” And yes, Vermouth and other aperitifs were served.

Confitería Congo was located at 717 Avenida Forest, and was frequently a site of dances during the Golden Era. The place did not look large from the outside, as can be seen in the background of this photo (taken in 1960 during an earthquake!)…

Buenos Aires summers are hot and humid, and air conditioning did not become common until the late 60’s and 70’s. To deal with the summer heat, Confitería Congo opened its “jardín de verano” (summer garden) which had a 54,000 square foot open-air dance floor. This photo from 1932 shows what one of those outdoor summer dances looked like…

The remains of this garden and open-air dance floor can still be seen from aerial views; although the location has since been converted to a technical school…