What are the mosquitoes & sandflies going to be like in August in Playa Dorada?

What is the best defence against them, and what should I use after I have been bitten? Are they going to ruin my holiday because it is the rainy season and there is a big still water lagoon between my hotel (Blue Bay Villas) and the beach?! I heard that the locals use Vics Vapour Rub on their bites...?