you do not mention your booking class...
there is one meal on this flights brakfast, lunch oder dinner depending on departure time
if you go for Indian vegetarian option, one of the several special meals, then you have no other choice than what they serve you

If you do not choose a special meal, there is usually a vegetarian choice on flights to/from India but not to SIN (I do not know any Chinese or Thais, who are vegetarians...so I guess percentage is much lower than among westerners)