Hi Megha, You can take the 3 months tourist visa, coz that'll be a safer option :) You might feel like extending your stay, then you won't have to worry much about it.
Acquiring 3 months single entry visa is advisable for a hassle free travelling. An Indian can avail the facility of Visa-on-arrival. However an online approval from Vietnam embassy is required before leaving the country. Visited Vietnam early this year. Feel free to if any info required regarding Vietnam travel. Happy Travelling...!!