Before booking, you should get your expectations in line. As described in many of the reviews here, you are sharing the apartment with the owner. Yes, you do you have your own floor and bathroom, and yes, the terraces and kitchen are on the same floor as his living quarters. That being said, you are 100% welcome to use the terraces as you like. The rooms are clean, and bathroom, although small, is very clean. The people complaining about the narrow steps leading up to the apartment and small bathroom need a reality check - you are in Europe, and Vernazza has been around for hundreds of years.
As for the host himself, Enzo is the man. He invited me to have a glass of wine with him on the terrace when I arrived, and he even cooked me some lunch the next day with some fresh local anchovies in his grandmother's recipe. He also provides great recommendations on where to eat, not only in Vernazza but in the other 5T towns as well. He couldn't have been more friendly or welcoming. This place is a great option, especially if you are a solo traveler.