No, you cannot.
You are allowed to enter Vietnam later than the granted arrival date, but earlier arrival is not allowed. In case you change your plan and have to arrive in Vietnam before the granted arrival date, you are recommended to apply for a new visa.
Your Visa type is: 1 month single entry Visa
Entry date you apply: 28 Mar, 2015
>> You are allowed to enter Vietnam whenever you want from 28 Mar, 2015 to 28 Apr, 2015. And please make sure you will exit Vietnam before 28 Apr, 2015. (You can stay more in case you make your Visa extension).