"Since veganism is a choice by health nuts and animal rights activists, would it still be illegal to spike their meal with meat?
It would NOT be legal to "spike" a vegetarian's or vegan's meal with meat, and/or any other type of animal products or byproducts. It's actually possible for someone due to circumstances, not be able to eat meat, that's allergic to fowl/poultry, and fish/seafood/shellfish, due to a bite from a parasite, such as the Lone Star tick, to also develop what's known as Alpha Gal. As such spiking a person's food with meat, could cause a reaction, that can range from mild, to very severe, and severe enough, to even cause a person to die. In such a case, you can be taken into court on charges for the unlawful jeopardization of another person's health, and in the case of death, you would face at minimum involuntary manslaughter. There are even some for whom even the smell is enough to cause an allergenic reaction.
Also doing so, shows such an action as not only being disrespectful, a lack of both personal morals, and personal ethics. It also shows a lack of maturity. For myself, and as someone whose diet does contain meat, it also makes me question some of how you were raised. Practical jokes can be ok, however they're not ok, when it comes to what someone else believes, or what they eat, as well as in other areas as well. I can appreciate a good practical joke, but not the kind that can put others at risk, or go against what they believe.