Actually Vandyke (as all other iron processes) is pretty sensitive to humidity level in the paper; if your environment is not too humid odds are that you'll notice an increase in the speed of the emulsion too. (I mean if you don't wait that much. All iron processes except for pure platinum printing more humidity gives you more speed. It's the other way around with pure platinum printing.) My usual practice was double coating within 5-10 minutes, wait until the paper is surface dry (not completely dry!) and print that way. IIRC, the whole operation (coating + exposure, not processing!) was taking no more than 30-35 minutes... (I don't do Vandykes since maybe 4-5 years; I almost exclusively do gums or gumover cyanotypes - and pd, occasionally - now.)