1)I don’t use the A4988’s becasue they don’t seem to handle heat as well, but the vref should be twice what it needs to be for 1 stepper. That is dependent on your stepper so I can’t tell you what to set it at. You are going more for as high as the steppers and driver can handle not the bare minimum to get it moving.
2) Start as loose as possible and only snug things up if you notice an issue. They will self smooth after a few uses. You can also just model a circle as big as you build area and let it run that for a while to get things smooth.
3) you should not get belt skipping. Have a look at the beta values, I spent almost 2 days on these settings, they should be very conservative. Although it is set for 32nd stepping so you might need to find these values on your own using the sripts to make it easier.