5e Vecna

[Myad]

Is it possible to insert him into 5e forgotten realms setting?
and if so anyone know where i can gather informaition on it?

[Zacchaeus]

Yes, since it is your campaign you can do what you want. Information here (https://en.wikipedia.org/wiki/Vecna) and here (https://dnd4.wikia.com/wiki/Vecna) among many others.