Will I have to pay tax in India, if I transfer my salary from here to my India account?
Tax does not depend on whether you transfer your money to India or not. It depends on how many days you are outside of India. If you are outside of India for more than 182 Days in the given financial year; 1 April 2015 to 31 March 2016 then you are Non-Resident. If you are more than 182 days in India you are resident.
If you are Non-Resident, you will not have to pay tax on salary received. If you are Resident, then you will have to pay tax on salary [and not allowance] received in Vietnam
is it possible that I could deposit my cheque directly to my India account
Yes you can deposit the cheque in India. It depends on how the cheque is drawn up. It can take anywhere between 1 month to 3 months to get it credited to your account.