There are two issues for night vision, pupil size and retinal sensitivity. And it's only the second one that is really the night adaptation and is not affected by the red light.
First is pupil size. Bright lights shrink the pupil to let less light in. After a time in the dark your pupils expand to let more light in. Which is why when you turn the light off in your room at night you can't see much for a couple of minutes or so. And conversely why when someone turns the normal light on in the middle of the night it is wincingly bright. This is the pupil taking time to adjust. This process is just about the pupil trying to meter the strength and brightness of the light, nothing more.
Secondly is retinal sensitivity. After a few minutes of darkness, the proteins in the rods in the retina make it more sensitive to light, allowing more to be seen. This process to fully develop takes about half an hour. Exposure to long wavelength light (red) doesn't reset this process. So dim red light will preserve this night vision adaptation of your retina.
Turning your cockpit lights down will keep your pupil dilated and let a good amount of light in and help you see outside (the outline of a towering cu against the background of stars for example).
But if there are any wavelengths of light not in the red spectrum, (eg EFIS) then the rods in your retinae won't fully function in low light.
In practice pupil size gives a great improvement, the retinal sensitivity is the icing on the sake.