If you are flying with Singapore airlines, they offer several vegan options, although it is not explicitly stated in the description on their website.
The western vegan meal is obvious (VGML), so is the raw vegetarian meal (RVML). However, the rest are less obvious even though they stated dairy-free.
I wrote in to enquire about their Vegetarian oriental meal (VOML) and the Jain meal (VJML), and they have confirm that these two options are suitable for vegans, except that the Jain meal may be served with yoghurt separately from the main meal.
Anyway I had a return flight from SQ and got their Jain meal (which in my opinion is better than oriental meal).