Fishbase suggests that guppies are native to 9 countries, including Antigua and Barbuda, Barbados, Brazil (in the north), Guyana, Netherlands Antilles, Trinidad and Tobago, Venezuela and the Virgin Islands. These places are along the Northeast coast of South America and ranging north beyond Venezuela and Trinidad through the island chain known as the Lesser Antilles (also the Leeward and Windward Islands).
They aren't known from every Island in the chain. That could be because of natural causes or it could be that they were spread in the water barrels of merchant and war ships ranging north from the continent "way back when."
Perhaps just a typo, but it was startling to see guppies listed as native to Vietnam, in Southeast Asia. Even sources with pretty good academic credentials can offer mistaken information.
If Fishbase is to be usually trusted, habitat profiles show guppies to be found in brackish water only in Trinidad-Tobago. (An occasional skeptic might wonder how many of these animals were washed down stream in floods.) Salt water wasn't a listed habitat in any of the countries in which they were native. In most places they live in shallow, slow moving waters with some vegetation.
Food items included algae, insects (larvae and adults foolish enough to fall on the surface) and all sorts of crustaceans living in the water column. This is why mosquito control people have transplanted wild strains of guppies, also known as the millions fish, to all sorts of warm water areas around the world.
Although guppies don't have quite the evil reputation of Gambusia (a.k.a. dambusia) transplanted from the Southeastern U.S. so that they could eat fry of fishes all over the world and even helping to exterminate local species, several countries report an adverse ecological impact after the introduction of guppies. The guppies were eating others things (fish eggs? fry? The crustacean foods the other fish needed?) rather than just mosquito larvae.
Too often we just don't know how an introduced species will fair in a new habitat. A number die off. A number become a problem, lacking natural predators. Not a lot accomplish exactly what was intended.
Often, as in the case of mosquito control, they kill off or out compete the local creatures which dined on mosquitoes. It amazes me that presumably well educated wildlife officials and scientists wouldn't figure out that if a new animal was introduced into a habitat that they would take space previously occupied by other creatures!
Then there was the case of a Detroit aquarist, who saved and saved and flew half way around the world to collect annual killifish in Kenya. These annuals (the very beautiful Nothobranchius species) live and lay eggs in ponds which dry up for part of the year. Obviously the adults perish, but the eggs endure in the soil (much like daphnia, shrimp and - oh yes - mosquito eggs). When the rains come the Nothobranchius hatch out and gorge on baby mosquitoes. Where pollution, habitat destruction, introduced species or insecticides haven't killed off the killies, they often eat so many mosquitoes that mossie - carried diseases such as malaria are rare or at least much less common.
That aquarist arrived in time to find guppies in the Notho habitats. The guppies came in when rains flooded local streams. They undoubtedly fed on some mosquito larvae. But they may also have feed on the newly hatched Nothobranchius. When the dry season came the guppies and remaining mosquito larvae in the isolated ponds would again perish.
The next year the rains would again come. Mosquito eggs would hatch. Because the guppies had destroyed the Nothos, one less predator would hatch out in those ponds. If the rains didn't flood permanent bodies of water enough to wash guppies into the ponds, it would be an interesting year for health officials where public health budgets are extremely modest by western standards.
In the meantime, think of that aquarist who dropped a couple of thousand dollars only to collect wild guppies. If he wanted wild guppies, natural habitats (or Florida) were much closer to home.
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