You might actually be surprised at how much heat you wouldn't use by adding another vent actually
From the stuff I've read, humid air will actually rise higher than hot air (which technically doesn't actually rise - hot air just gets less dense as the air turns to vapour) so by the time the air turns humid you've actually lost a lot of the heat that the air once contained, and so getting rid of that air won't actually sacrifice a lot (if any) heat
FWIW I had an issue early on with over-humidity in my tank, I just pushed another vent through in the top corner of the cold side. Not only did this reduce the humidity by about 8% but it also actually made it a lot easier to regulate my temps. Now that just might be me so don't take that as gospel
Think of it this way instead - what's the worst that could happen by popping another vent in? If the worst case is that you lose too much heat you can block the vent back off and try something else, but it may be worth having a look
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