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.