So try not to be so smug.Lastly, I absolutely do not treat my clients' data security casually. 

Is this a secure strategy for decreasing vacancies?

Must haves - web based, so it can be accessed anywhere. 

It also helps should you want to secure financing from that bank in the future.  3. 

It has worked out extremely well with unbelievable returns in the 30-40% return rates not including the equity appreciation of now having secure long term tenants paying huge lease rates.

Is it legal to put a tenant's (or multiple tenants') security deposit into a savings account and then collect the interest that the deposit makes for you?

I use the three that I see most often on this forum: Rentometer.com, Craigs List, and real estate web sites like Zillow.

Is this violating any sort of securities laws?

In doing this, their money is secured by both a hard asset and a personal guarantee. 

Think earnest money, rents before they are disbursed to the client (you) & tenant's security deposits.