Vacci viene composto di due parole: Va (l'imperativo della seconda persona del singolare del verbo andare) e Ci (particola pronominale o avverbiale, che, in questo caso, si riferisce a "con el sale"). The meaning is: Don't consume too much salt, please!

grazie. Chiarissimo

I can tell you with 100 % certainty (demonstration will follow) that here /ci/ means /the thing where you are adding the salt to/, / la cosa, il posto, al quale si sta aggiungendo il sale /, quindi si tratta di un pronome di luogo. Proof: If you enter a kitchen and abruptly say to the cook: / vacci piano con il sale /, the cook (if not angry beacuse of your order) will almost certainly ask you ? / vacci dove ? /, as the context, the /where to put salt/ is not clear. But if you say the same thing, while the cook is putting the salt into the pot for spaghetti, than he could just say /I know my job, thank you!/, in Italian: /conosco il mio mestiere, grazie!/, as /-ci/ is linked here to a clear context of place!