Income tax filing query in case of overseas salary

My Clients is an Indian PAN holder in FY 2018-19

01/04/2018  to 19/08/2018     he will be in India                                                                         Salary will paid by Indian Co.

20/08/2018  to  15/11/2018    his Indian co will depute him to Vietnam on  BUSINESS Visa  -  Salary will paid by Indian Co.

16/11/2018  to   30/04/2019  his Indian co will depute him to Vietnam on    WORK Visa        -  Salary will be paid by Foreign Co.

Query-

1. Where he will Filed Income Tax Return in India   OR  Vietnam

2. TAX Benefit  ?? ,  In Vietnam Tax is 35% 

3. In above whether he will be RESIDENT or NRI

Please guide me