About digging nails:
Digging a lot of nails with the Vanquish 440 is mostly an ear training issue. When I first started I dug tons of iron because I couldn't differentiate the sound of a good coin signal from the "iron falsing" of a rusty nail.
The rusty nail signals are not as repeatable as good nonferrous signals. When you have identified a possible high-tone coin signal, sweep over it steadily and slowly back and forth. A nail signal will not repeat on every pass--it feels like you have to coax the good sound out of it. If you sweep over the signal from a different angle it will still be inconsistent or will disappear.
The good high-tone coin signals will generally repeat on every pass from 360° around in clean ground. In iron-polluted patches of ground (e.g. yard of demolished house), coins and other nonferrous targets may have sketchier signals due to close proximity to iron. For example, a wheat penny might only sound good and repeat from one direction. So in these irony areas it is useful to detect from many different directions to maximize the finds.
I would recommend trying to train your ears in pretty clean ground at first. Just get used to digging clad coins with solid signals. Once your ears know what to listen for, the rusty nail "imposter" signals will sound less appealing.
Once your ears have some experience you can move to the more iron-polluted areas (if you want to). The trashy/irony areas can be great because I think they scare away some detectorists, leaving lots of things still in the ground. I like to use coin mode in all metal (horseshoe button activated) in these areas. I like it because good high tone coin signals will stand out from the iron grunts, and coin mode will give you the best "target separation" or "recovery speed". But running in all metal in iron trashy areas can be overwhelming (sensory overload) until you are used to it.
About the sensitivity: this is just my experience, but I can run full sensitivity nearly 100% of the time with no issues. Even at full sensitivity the machine is very quiet and stable unless you are near a power line or another source of interference. Dropping the sensitivity does reduce the iron falsing but at the expense of some depth. But if your ears can tune out the iron falsing (keep practicing), full sensitivity will give you the most information.