I don't understand how to distinguish between alzheimer's and vascular dementia?

Not easy: Clinically, alzheimer's tends to be a smooth decline over time, while vascular dementia declines in steps, as small regions of the brain lose function. Neuropsychological testing may lend evidence for, but not prove, one cause or the other. A definitive diagnosis can only be made by brain biopsy (which is rarely done), or at autopsy.

The course of these: Dementias is quite different as are the causes and treatments. Vascular dementia exhibits a stepwise course while alzheimer's is characterized by a slow and steady progression of memory loss.