We can’t know for sure. But there have been several examples of marine mammal species that have come back from the brink of extinction, such as the northern elephant seal or the grey whale (in the North Atlantic). The grey whale was actually declared extinct in the 18th century, before recovering to healthy population numbers.

The vaquita population can recover if the immediate threat is removed from its habitat. And the only real danger for the vaquita are gillnets.

People ask me all the time, well, if there’s only 60 [now 30] left, can we really save them, can they really come back, and can the population survive? And I would say, the answer is probably yes.
Dr. Anna Hall, Porpoise Conservation Society

In the fall of 2018, observers spotted live vaquita, among them at least one calf. There are still vaquita there, and they are still reproducing. If the threat of gillnets can be removed by eliminating the illegal tototaba fishery, the vaquita might have a chance.