Dear Bala Kumar,
Thanks for your contact!
If you are Thailand resident, please inform that Thai passport holders are granted 30 days for Vietnam. Therefore, if you stay in Vietnam less than 30 days, you are free from a visa. If you are not, you have to apply for visa to Vietnam.
Regarding visa validity, we are pleased to inform you that the date of arrival you submit in visa is important regardless of the date of applying.Thus, to avoid any trouble relating to visa validity, your proposed arrival date in visa should be the earliest you plan to arrive.
For example: if you apply for 1 month single visa and you submit arrival date is 15 April 2013, your visa will be valid from 15 April 2013 to 15 May 2013. You can enter Vietnam anytime between 15 April 2013 and 15 May 2013 but must leave the country by 15 May 2013.
We do hope that this information is helpful for you.