Looking for a vacuum tight valve. The requested specific leak tightness for the entire tank should be below 158 * 10-9 mbar * dm3 / s (for air). The volume of the tank is approximately 10 dm3.
as far as I know, in general terms, "ordinary" industrial valves have leak rates, through the stem packings and body flanges, in the order of 10-6 mbar*l/s and above (considering "fugitive emission" helium tests); along the line, i.e. through the  ball valve seats, usually the flow is not of the molecular type anymore and leak rate entities may be also some decades greater.
Then, if you need a maximum global leakage not exceeding 10-7 mbar*l/s, I would suggest to search for "special" dedicated valves for vacuum services and/or to install two shut-off valves in series (possibly with two different seating mechanisms).
Of course other aspects must be considered to select the proper valve and options (for example: rubber vs metal seating; rising vs rotating stem; live loaded packing vs o-rings; etc...): among them the service temperature, the vacuum level (in terms of absolute pressure) to be reached and mantained within the tank, the valve size, the chemical nature of the service and the frequency of operation.
Perhaps some more information could be useful to better advise you.

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