A few comments:

Hindu meal is in theory not necessarily vegetarian, but the Indian Veg meal is. Hindu meal definitely excludes Beef, but may include chicken or seafood. That said, in practice the two meals are often combined into one. Similarly, Indian and Asian veg are sometimes combined together (eg breakfast service out of BKK) and sometimes all are replaced by a fruit plate (domestic snack services sometimes do this).

Indian veg meal is definitely not a VEGAN meal-- it often contains paneer, an Indian cheese which is used in dishes with spinach or other vegetables.

It is difficult to make generalizations about domestic v. international. In my experience, it depends on catering station. My general eval of the Indian Veg meal out of various major stations is as follows: (haven't done Asian regularly in a couple of years so won't say). Unless otherwise specified, this applies to lunch/dinner flights only and the meal consists of rice with several Indian curries.

HKG, LHR, FRA: Outstanding
NRT, JFK, LAX, PHL: Very good.
SIN (breakfast only): Very good (Indian breakfast).
SFO: passable.
IAD: inconsistent, ranging from very good to passable.
ORD: inconsistent, ranging from passable to disguisting. My last flight from ORD-KIX last month was miserable-- I got roasted potatoes and steamed zucchini as my entree.
EWR, HNL: poor
DEN, PDX: never really an Indian meal, always replaced by a fruit plate snack with nuts sprinkled on top in my experience-- which is not too bad if you're not hungry.
BKK: "Indian" Breakfast consists of oily spring rolls and stir-fried vegetables. Disguisting.

Hope that helps.