It already was. Scandinavians would stay in Greenland until the 15th century when a conflict with the natives would finally have them expelled for good, also not returning because of temperatures dropping. They however did leave knowledge of very far away lands lying to the west not to mention how they were responsible for leaving a North Sea and Baltic Sea seafaring tradition that had connected trade with the Mediterranean since the end of the Viking Age at least.
He had a copy of both Mandeville and Marco Polo, alongside Pliny the Elder. And no, Mandeville wasn't more influential. He doesn't mention Mandeville in his diary, only Polo, and the way he speaks of the Great Khan and Cathay is straight from Polo. Not that he wasn't influenced by Mandeville, but he was far more influenced by Polo.
It's difficult to say really. Marco Polo's writings were one of the
best sellers of the Middle Ages, but the Vikings had gone to the Americas without any incentive of finding India or Cathay centuries before Polo's stay in China, and there was already a tradition of unimaginable riches being found in India dating all the way back to Herodotus, who said it was the richest and most populous area of the world, and being popularised by the Alexander legend which had Alexander finding what are basically early versions of El Dorado in India or close to it.
What is true is that the exploration of the Canary Islands and their subsequent annexation by Castile started because the Vivaldi brothers, two Genoese explorers from the 1290s, decided to see if they could circumnavigate Africa and reach India, or reach India from the west, inspired by Marco Polo's writings, and instead ended up dying somewhere in the Moroccan coast. Another explorer by the name of Lancelotto Malocello went looking for them but didn't find them and decided to stay in one of the Canaries, the island that is now called Lanzarote after him. It is this that took the attention of Portugal and the kingdoms of Castile and Aragon and which started the subsequent explorations of the African coast and ended in the development of the transoceanic carrack and caravel.