I guess it means the valve shall seal when pipe pressure becomes lower than valve external pressure (usually athmospheric pressure).
In such case, differential pressure is reversed. You shall insure your valve sealing arrangement works in reverse way too. It is not the case for lip seals or chevron type seals.
Particular care shall be also considered with O'rings in high pressure designs.
For standard Gate valve, I don't see any problem as far as packing is design for.
Gate and globe valves are not generally vacuum tight or may leak after some time in service. In order to prevent air in-leakage, a water supply is piped to the atmospheric-pressure side of the valve to provide a water seal. (Most installations are in a vertical run of pipe.) A level control may be used to automatically replenish any water leakage through the valve into the vacuum space.
In addition, a "lantern ring" may be used to prevent air leakage through the stem packing. The "lantern ring" also requires a water supply.
In the past, there is Royal-Dutch SHELL MESC specification for vacuum service. Afterward, the end-user expired this specification since the most of gland packing/ seal can withstand reverse pressure, i.e. from atomospheric to valve internal in vacuum condition. Unless otherwise the external pressure of the valve is higher than atomospheric, I think this reference can be applicable to your situation.
If your situation is more complicate, I would adivse you to tell members more details of application. Perhaps, I can check more concrete condition to close to your expectation.
