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

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.
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.

Related Questions

I was wondering what are the differences between Alzheimer's and vascular dementia?

Significant. Alzheimer's is a progressive dementia characterized by short term memory loss and another domain such as naming that continues gradually. Vascular dementia is a step wise (non progressive) dementia caused by strokes, where each stroke causes a cognitive loss which is less clearly demarcated since it is more subcortical. So patients have problems with recall/retrieval of info or other dysfunction.
Briefly, Ad progresses slowly, but it does progress. Vd occurs suddenly and stabilizes. With additional incidents, it again abruptly worsens, then stabilizes. Depending on the site of the damage, motor function will or will not be involved. Ad may take a long time to effect motor ability. It's a bit like "disease vs injury". Below is a link since 400 chars is too little. Hope it helps.

Is risperdal. 5 mg dose safe for an elder patient with alzheimer's and vascular dementia? I read this drug can cause sudden death in elder patients.

No. According to all studies so far, any antipsychotic used for the purpose of treating behavioral problems or psychosis related to dementia is associated with earlier stroke-related death. This occurs regardless of the antipsychotic and the dose. There are some studies that have shown some antipsychotics to be safer than others (like quetiapine) but those studies are small and inconclusive.
Depends. Every decision about using a medication depends on the specific risks and benefits of the particular patient. Symptoms are already occurring and may have minor or major risks to life, health and quality of life. Side effects are possible though not definite. Discuss your questions with the prescribing doctor to better understand this specific situation.

Are vascular dementia and alzheimer's essentially the same disease?

No. Alzheimer's dementia is characterized by plaques within the brain cortex. Vascular dementia is because a blood vessel is clogged resulting in decreased blood flow to an area of the brain and the damage to that area of the brain.
No. Vascular or multi-infarct dementia and alzheimer's are distinct disorders. Vascular dementia is due to blockage of blood vessels supplying the brain. The blood vessels are generally not affected in alzheimer's, though as alzheimer's occurs in old people, they may have co-exiting atherosclerosis.
No! Vascular dementia is a problem with the blood vessels in the brain that cause brain cell death. Alzheimer's is caused by the development of plaques in the brain.