Being that no ones started any sort of online campaign asking them to do as such (for old series and for gintama/reborn), I dont know if there is as much of a "fanbase" that wants that as you say. [Also wouldnt gintama and Reborn need a new translation, thought people hated viz's handling of it back then???]
Also the vault is only 1.99 a month, none of us really know how its profitability goes, and isnt it hard to find and get manga from before the 90's? At most 5 are announced a year from the entire eng industry at large, several old series failed kickstarters, or simply never came out due to challenges. Incredibly old series might be such a super niche market that it may not be feasible at the current setup of the vault (I'd think if the vault were doing THAT amazing, then Viz would have expanded it to all their manga...)
Also I think they ought to focus on getting more form JUMP+ and etc before going to old series, the lack of choosing everything there or even whats on Mangaplus ought to be telling of the state of the viz jump vault platform.