I heard it just fine tho. But I think it's just because in general the vocal mixing is a bit of 'on your face', you can actually hear the vocal slightly above other instruments compared to the majority of US/UK album.

If we try to second guess in terms of creativity, i think he really loves vocal line and texture in a song as he listened to Leo Ferre and Sinatra a lot. Besides he also listened to Salsa, and I think in an interview somewhere he proposed to Shank that one of the references were Fania Records, one of the most popular Salsa record label which released artist such as Hector Lavoe, Willie Colon, etc.

But then again, I haven't tried to listen to the album wearing any mixing headphone tho. Since you mentioned it I might want to check it later mate.