A proper vase will hold water, so this vase will have to be modified so the base instead of having a 2 mm wall will be solid, and the bottom of the body and foot closed so it can hold water and print. This involves selecting the internal walls of the vase, which can be difficult to do without selecting the external walls as well, so a new selection operation will the used; Select More:
Apply the Solidify modifier.
Toggle to the Wireframe view on (Z).
Enter the Edit Mode (Tab).
(De)select All (A).
In the front view, zoom in on the base and identify a line of vertices, where the noses meet which are on the inside and carefully Border Select (B) the whole line. Try to identify a line that is not so close to external lines in this view to make the selection easy:
From the menu at the bottom of the View Panel, click on Select More or...