Vines growing to the top of the tree and covering the leaves cut off the photosynthetic process that is vital to the tree. Photosynthesis creates carbohydrates that travel throughout the tree and to the roots. Without this, trees will weaken and die. Vines that cover the bark of the tree block lenticels through which gaseous exchange takes place. Without this exchange, cells in the area will die and weaken. Excessive vines around your tree could also be covering up decay, disease or insect infestations.
For more information check out the U.S. Forest Service Invasive Species, and the Indiana Invasive Species Council.