The way vegetation is officially named, classified, and identified has critical implications for ecosystems and biodiversity conservation. Yet little attention is given to how such issues hinder the efficacy of laws mandating environmental conservation on private land. In the Brazilian Amazon where half of the land is now already under private tenure or is available for future land-uses, differences in vegetation mapping and interpretation directly affect the level of protection in private rural properties, especially in transition areas where forest and savanna areas intermingle. Since Brazil's Native Vegetation Protection Law (NVPL) attaches a higher percentage of protection to forest-located properties, landowners may be tempted to use conflicting mappings and different vegetation classifications to claim their properties are located in areas other than forests to reduce their conservation requirements. In this paper, we compare three official vegetation databases and examine different law interpretation scenarios to assess the extent to which the level of private conservation may fluctuate. We found a difference of up to 430,000 km2 of protected vegetation (an area the size of Iraq) according to the database and vegetation characteristics chosen. This technical ambiguity may lead to make additional deforestation legal or reduce sharply the amount of vegetation to be restored for these areas, if left unaddressed. Clarifying the database and criteria used to define forest is critical, especially as Brazilian states may make different choices in that regard, and cases in which loopholes are exploited occurred in the recent past. Given the importance of this region for global biodiversity conservation and climate, we highlight the urgent need to: (1) support additional research to clarify vegetation characteristics and location; (2) agree on a harmonized methodology to determine forests for NVPL implementation, and (3) explore alternative criteria for defining forests when databases conflict.