Vicious means that something is really mean or nasty. Imagine if someone takes your favorite toy away from you and won't give it back. That's vicious behavior!
On the other hand, delicious means that something tastes really good. Imagine if someone gave you a piece of chocolate cake. If it's really yummy, you might say it's delicious!
So, when we talk about something being "vicious and delicious" it means that it's really mean or nasty, but also really tasty. This doesn't happen very often, because most people don't find mean things to be enjoyable.
It's kind of like if you had a food that was really, really spicy. Some people might say it's delicious because they like the heat, but to others, it might be too vicious because it hurts their mouth.
Overall, being vicious is never a good thing, but enjoying delicious things is definitely something we can all relate to!