The rest is optional: either get as many provinces as possible, or money to pay off loans. In the peace deal, try to secure the connection from the island to your mainland holdings. If Vijayanagar maintains naval superiority and gets their troops onto the island, it might be impossible to stop them from sieging player's capital, thus inflicting heavy war score loss and war exhaustion. If Jaffna was already annexed before the war, just one regiment on Lanka should suffice. It's possible to split armies, having one force to defeat Jaffna, while the rest tries to take and hold the other side. The most important task is to secure the strait from mainland to Lanka. Also, naval superiority might once again be a problem, unless the player invested large sums of money into naval forces. Player's armies, together with their allies', will probably be stronger, but disunited - indian allies will have to spend some time sieging Vijayanagar's forts before they can reach the player. The first war is expected to be tough, and it might be necessary to go into debt and hire some mercenaries. Bahmanis is probably the best option, as they rarely resist an opportunity to attack their rival. In the meantime, the player should also secure some stronger alliances against Vijayanagar, which is both their immedeate threat and expansion path. One way to bypass it is by temporarily allying one of the minors, asking for military access and placing troops in their land before declaring war.
The player should be able to get together enough troops to defeat them, however, unless they spend significant amount of money on ships, they might have problems reaching naval superiority and thus securing passage from Lanka to India. It's heavily recommended to start with south India minors before Vijayanagar conquers them all. With Lankan holdings secure, the player needs to start expanding into mainland. Sometimes even Vijayanagar can be asked for help then you'll have easier time maintaining naval superiority and defeating Kotte's allies on the mainland. With some luck they'll even be caught drilling. Kotte doesn't usually get many allies, so going temporarily over the force limit should be enough to win the war. Also, it is not much harder - the player has to do only one extra step by defeating and annexing their overlord. However, starting as the vassal instead gives the option to get another achievement along the way: Cotton Kandy.
Choosing Kotte is generally easier, since they start with Kandy as a vassal. Both share the same (sinhalese) ideas and neither has unique mission tree. The player has to choose from either Kotte or Kandy, the only two theravada countries in India.