I would like to understand what is the ethical, theological etc background for vaishnavas why we should not eat anything contaminated with non-veg ingredients. Of course, I avoid any possible connection, but it is so difficult that you can practically forget 100% purity, unless you allow for a certain grey-zone. Even if you eat only prasad made from scratch, you are likely to have some ingredient that has come in touch with non-veg substances, esp if you include also the process of cultivation (animal ingredients in fertilizers used). If you buy flour to make bread, there are anti-clumping ingredients made of cow-bone. Even if you buy organic wheat, it is likely to have been grown with some kind of fish-bone ingredient in the fertilizer etc.
Cow bone is not only in the food, it's also in the paper we use, in the clothes, in colour, in everything.
So the ethical questions are of importance:
Is it that we are participating in the slaughter process, that such food should or could not be offered to the Lord, that it is impure, or which other consideration?
Another reason why I am asking is that the societies we live in are dependent on the system of cow slaughter. Milk products would have to be forbidden for us too, only Vegan life-style would be an option. Even then we continue to implicitely support the agri-policy of cow-slaughter. If you look thoroughly at how the agri-business works and how our societies are dependent on the policies operating in it, we cannot close our eyes from the fact that we are also in the net of its interdependences.