Vacuole Function. The Vacuole is a cell organelle that is present only in plant cells. The main vacuole Function is in maintaining the structure and shape of the cell. Cells of plants do not increase in size by expanding their cytoplasm but by increasing the size of their vacuoles.he most useful, and the largest, vacuoles are mostly found in plant cells. Vacuoles are filled with a different liquid called cell sap, instead of protoplasm. The sap’s composition is primarily water, but this can vary from one cell to another and even from one vacuole to another vacuole.