The decanter shown in this post was definitely supposed to have a stopper. Venini often did not grind the interior of their bottles, so that is no way to tell if a piece had a stopper or not. The only way to tell is to know the venini forms. A look at the Venini catalogues will show that this particular shape, with its narrow neck and flattened rim, was never sold as a vase.
Generically, if the interior of the neck is ground then the piece was made as a decanter. If it is unground then you have to know the item or company in question.