No one considers Vasco da Gama to have discovered India. It is half baked knowledge. What is acknowledged is that he found out the sea route to India via the Cape of Good Hope.
The traditional European Trade to India was via Constantinople, that is present day Turkey or through the mediterranean trade via Egypt. Two events cut out this opportunity for the Western European Merchants. These were the rise of the Islamic Ottoman power and the monopoly of the merchants of Venice and Genoa over the mediterranean trade.
The Ottoman Empire and the christian Empires of Europe were largely antagonistic towards each other. This meant that the land route option was closed to the merchants who would have had to traverse the perilous routes through hostile territory. Trade by its very nature is risk averse. Hence merchants wanted an alternative.
Meanwhile, the Italian Merchants were reaping a fortune with their near monopoly through the mediterranean trade. In those days goods reached Egypt via sea and then overland to the ports of mediterranean and then Europe. Italians were not willing to cede their monopoly and Western European nations had to pay a huge price to the goods from India which were in great demand.
The Kings and merchants in these countries, entrepreneurial by nature, found an opportunity in finding an alternate route to India. They could break the Italian monopoly, avert the Ottoman risk and still reap huge benefits if they could find an alternate sea route. The leaders in sea voyage were Portugal and Spain in those days and both set out to find a sea route to India.
As it would turn out, Portugal would be one to succeed with Vasco Da Gama able to cross the cape of Good Hope and finally reach India. It is said that he actually did little finding on his own. He crossed cape of Good Hope (The route to which was found by Bartholomew Diaz) and reached till Madagascar where an Indian Merchant helped him to reach India. However, he still is considered a pioneer for what he did.