In recent years, accumulating evidence has suggested that vascular risk factors contribute to Alzheimer disease (AD). Vascular dementia had been traditionally considered secondary to stroke and vascular disease. It has been traditionally distinguished from AD, considered to be a purely neurodegenerative form of dementia. However, in light of this more recent literature, it appears that there is a spectrum: ranging from patients with pure vascular dementia to patients with pure AD and including a large majority of patients with contributions from both Alzheimer and vascular pathologies. In this article, we discuss the impact of vascular risk factors on AD and its consequences at the individual level and at the population level by highlighting the concept of attributable risk. We then discuss the key questions and next steps involved in designing a therapeutic trial to control vascular risk factors for the prevention of dementia.