Visiting Vienna in September

Hi everyone.

I am Japanese American engineer living in Frankfurt and plan to visit Vienna sometime between 9/ Protected content my mother who is visiting from Japan.

My questions are as follows:
Is staying 4 days/5nights enough to see Vienna and tourist spots?

What is the no.1 major attraction(s) in the city I definitely should visit?

Which part of the city I should stay in order to get around the city easily? My mom is has had some health issues recently, so access to public transportation is a must.

After Vienna, we might go to Salzberg. Is it worth heading out there with all due respect?

What is the best restaurant in the city without breaking the bank?

What is the best bar/nightclub in the city?

Which part of the city I should avoid going at all cost due to crimes and drugs?

Regards,
Steven