By Belén Carreño and Sarah N. Lynch MADRID/WASHINGTON (Reuters) -Spanish police on Monday impounded a superyacht belonging to Russian oligarch Viktor Vekselberg on behalf of U.S. authorities, the first time the United States has seized property belonging to a Russian oligarch since its invasion of Ukraine in February. Valued at more than 90 million euros ($99 million), the 78-metre-long “Tango” (255 ft) was seized in a shipyard on the Mediterranean island of Mallorca, the Guardia Civil police and the U.S. Justice Department said in parallel statements. The Spanish police added that documents a…