PARIS—Shares in France’s Lagardere surged 20 percent on Thursday after media group Vivendi said it would buy another stake in the company, paving the way for a full takeover. Canal+ owner Vivendi, which already owns 27 percent of Lagardere, said it had agreed to buy Amber Capital’s 17.9 percent holding for 24.10 euros per share, or about 610 million euros ($718.95 million). Vivendi said it would later launch a full bid at the same price once it has passed the 30 percent threshold requiring companies in France to make a takeover offer. “We believe Vivendi is primarily interested in Lagardere’s International Publishing business which would bring scale, synergies & savings to Editis”, analysts at JP Morgan said in a note. They also said that Vivendi would likely seek to sell the travel retail business to fund the acquisition. If successful, the acquisition of Lagardere will mark the end of what …