I took away some food.
When I asked the staff about the vegan-friendliness of their plates; the staff mentioned that they do not use any animal or animal-derived product; hence, they are vegan.
However, I did notice that in the allergen list they have, they write yoghurt for one of the desserts.
I did not check the list thoroughly, and I don't really know how up-to-date is that list; but if someone else could double-check on this issue, it would be great.
As the food itself, it was nothing extraordinary; but it was good. All the plates they had in the buffet (many choices!) were vegan according to the staff.
The prices are unbeatable (especially the take away ones), and the location is also good.
All-in-all, 100% vegan or not, I think it's a good place to stop-by if you're in Luxembourg for dinner.