There are three Viking articles in the Technical Archives section of the Club website https://rover200.org.uk/members-login/viking-tech-index about fuel tank repairs and operation .
Apr-Jun 2018 p18-22
Apr-Jun 2019 p22-25
Jul-Sep 2020 p22-24
The design of the breather system appears to have changed with catalyst cars. The vacuum is probably intentional to prevent petrol fumes leaking from the tank and making a smell. If there isn't a vacuum, it may mean there's a leak somewhere. I found the filler pipe and the associated breather tube were quite corroded on my 1990 214 when I took it apart.