That report comes from sources speaking to ModernWarzone, which claims that that name is very much a working title, and that this year's game may eventually ship under a different name. While it's worth taking this rumor with a pinch of salt, especially in lieu of any official comment from Activision or Sledgehammer, the title could provide a hint as to the nature of the game.
Whether or not the word 'Vanguard' features in the final title, it seems increasingly likely that we'll be returning to World War Two. Sledgehammer was the lead developer behind 2017's Call of Duty: WWII, so could easily be set to return to that period. What's more, a Call of Duty leaker has already suggested that this year's game will be set in World War II, and ModernWarzone also reports that some of the game's cutscenes will be set in the 1950s, soon after the period in which the war took place.
Activision has confirmed that this year's entry in its FPS series will be coming in late 2021, as is pretty much custom at this point, but we've had very little in the way of official information since then. That said, Call of Duty's reveal events tend to be in the summer, so we should find out more relatively soon.
Update: More information regarding the next Call of Duty game has reportedly surfaced.
Following its reveal of what it claimed is the working title of this year's release, leaker ModernWarzone has released a new report outlining the game's setting. In a recent video, ModernWarzone stated that "the entire game takes place in an alternate timeline where 1945 wasn't the end of World War 2," and will take place in the 1950s.