In these parts, our predominant POCO actually prefers 3/8" studs with nuts for them to make their own final connections. They don't want lugs at all. They even supply meter sockets for 320A and below for residential use at no charge. They do the line and load side terminations using Hypress terminals, so all we do is leave the cable hanging there for them. With overhead services, they even furnish & install the upper part of the SEU cable on 200 amp services or smaller.
I must say that I've never heard of lugs rated for 400 amps that are only secured by single Phillips screws. I'd say maybe a 1/4" flat head that requires a #3 screwdriver might suffice, but I agree that the vibration issue would leave me wondering as well.
"But the guy at Home Depot said it would work."