As assets of more than $4 billion change hands in a burst of restructuring in U.S. power generation, a European company is seizing a major piece of the action. France's Vivendi, an acquisitive communications and environmental services group, in a $1.7 billion deal, is taking over 23 power plants in Pennsylvania, New Jersey, and Maryland. Through Sithe Energies Inc., its 60%-owned U.S. independent power-generating subsidiary, Vivendi is buying the non-nuclear assets of General Public Utilities. Vivendi also is betting that a powerful position in the U.S. market will boost its chance of being an important player in France, where the energy market is slated to be deregulated in 1999.