My husband thinks you train a dog to be a perimeter guard dog by spraying him in the face with vinegar. Is that true?

My husband and I are opening a shop. My husband wants to get a dog that will offer some protection when we are not there. He is talking about "perimeter training" the dog. I think by this he means that the dog will guard the property in the night. He says to train a dog to do this, the trainer uses vinegar and sprays the dog with it. He says this is only done at night, so that when customers come into the shop during the day the dog will not attack. Is this the proper method to use? And can you train a dog just to attack at night? (Our shop will be open in the evening when it is dark outside.) I should mention we have a small child and I am concerned for her safety around a dog that has been sprayed with vinegar. Another question I have is if we leave the dog in the shop at night he will be alone for at least twelve hours every day. Isn't this too long? I should also mention the shop area is not fenced, and there are other shops in the same block.

Rhian