The 5e PH redefines portfolio to be the collection of domains associated with a god.
This is confusing for D&D players of past editions (or those referencing god info from prior editions) because portfolio in the past was used to describe the deities' associated areas of concern.
So in
3e Deities and Demigods page 58 it lists the 3e D&D pantheon's various portfolios and Vecna's portfolio is "Secrets, Intrigue"
In 5e Vecna is the god of secrets and his portfolio as a Greyhawk god is the knowledge domain while his incarnation as a Dawn War pantheon god of evil secrets in the DMG gives him a portfolio of the Death and Knowledge domains.
Vecna has not changed narratively between 3e D&D pantheon and the 5e PH greyhawk god description of him. The game terminology for portfolio has.
The 5e PH only uses portfolio to mean a god's collection of domains.
5e PH page 59: "All the domains over which a deity has influence are called the deity's portfolio. For example, the portfolio of the Greek god Apollo includes the domains of Knowledge, Life, and Light. As a cleric, you choose one aspect of your deity's portfolio to emphasize, and you are granted powers related to that domain."
Page 62: "Gods whose portfolios include the Tempest domain-including Talos, Umberlee, Kord, Zeboim, the Devourer, Zeus, and Thor-govern storms, sea, and sky."
Page 293: "The gods of Greyhawk come from at least four different pantheons, representing the faiths of the various ethnic groups that populated the continent of Oerik over the ages. As a result, there's a great deal of overlap in their portfolios: Pelor is the Flan god of the sun and Pholtus is the Oeridian sun god, for example." Looking at the list Pelor and Pholtus both have the light domain as one of their either one or two suggested domains.
The 5e DMG sometimes uses the term portfolio differently in the few times it uses the term.
5e DMG Page 10: "Each deity in a pantheon has a portfolio and is responsible for advancing that portfolio. In the Greyhawk setting, Heironeous is a god of valor who calls clerics and paladins to his service and encourages them to spread the ideals of honorable warfare, chivalry, and justice in society. Even in the midst of his everlasting war with his brother Hextor, god of war and tyranny, Heironeous promotes his own portfolio: war fought nobly and in the cause of justice." Hextor and Heironeous both have only the War domain as their suggested domain in the PH appendix entries for them, so the DMG here is using portfolio not as the War domain, but possibly as the domain interpreted through Heironeous's narrative elements.
Page 11: "Mystery cults often revere sun and moon deities and agricultural deities-gods whose portfolios reflect the cycles of nature."
Page 12: "The deity of a monotheistic religion has an extensive portfolio and is portrayed as the creator of everything, in control of everything, and concerned with every aspect of existence." and "Deities in a dualistic system maintain large portfolios. All aspects of existence reflect the dualistic struggle, and therefore all things can fall on one side or the other of the conflict. Agriculture, mercy, the sky, medicine, and poetry reside in the portfolio of the good deity, and famine, hatred, disease, and war belong to the evil deity."