Perry's Chemical Engineers' Handbook, 7th edition, now has a Process Safety section (26). Among other topics, there is one covering Hazards of Vacuum. They cite the following study which was made to allow prediction of the rate at which air must enter a tank with and without internal condensation to prevent a pressure difference from arising:
Influence of Product Vapour Condensation on Venting of Storage Tanks,
Chemical Engineering and Processing, Volume 22, Issue 3, November 1987, Pages 137-144
D. Fullarton, J. Evripidis and E. U. Schlünder
published by Elsevier-Sequoia
The study focuses on vessel inbreathing due to weather effects and gives the background on how the method was developed as well as an example problem to follow.
I don't have a lot of experience with "vacuum relief" due to condensation but the article seems reasonable and would offer a more rigorous approach to determining your nitrogen requirements.
If interested and your local library doesn't have the reference, you can obtain it from Science Direct for a small fee
The following is a link that will take you directly to the article abstract but I'm afraid you will have to copy the full address (between the quotes) and paste it in your browser address box
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You will see some text in German but the article is in English.
As an alternative you may be able to obtain it from the following for a lesser fee though you may have to wait a day or two before delivery.