The VIP requirements are:
* you must be a regular
* you must have 50 or more posts
* you must have 10 or more reputation
* you must have at least one post in the last week
* you must have at a ratio of at least 0.8

'Ratio' is defined as X / Y, where:
X = ((the total positive rep you've got) minus (the total negative rep you've got))
Y = the total number of non-neutral rep scorings that you have got

Therefore, if you have 10 reps, all +1s, you have a ratio of 1: ((10 - 0) / 10) = (10 / 10) = 1.

If you have 12 reps, of which 11 are +1s and one is a -1, you have a ratio of: ((11 - 1)/12) = (10/12) = 0.83

If you have 20 reps, of which 15 are +1 and 5 are -1, you have a ratio of: ((15 - 5) / 20) = (10 / 20) = 0.5.

If you have ten +1s, two +2s, and a -3, you have a ratio of: ((14 - 3) / 13) = 0.84

If you have fifteen +1s, a +2, a +3, two -1s, a -2 and a -4, you have a ratio of ((20 - 8) / 21) = (12 / 20) = 0.6

Put another way: for every ten points of non-gray rep you have, one of them can be negative. So, someone with 20 non-gray reps, of which 2 are negative, would be eligible for VIP. On the other hand, someone with 20 reps of which 3 are negative would not qualify.

Yes, it is tough to maintain a ratio of 0.8 or higher.

It is possible, however. In fact, there are over a dozen people with a ratio score above 1.

How did they manage this? They have more reputation points than they have reputation scorings.

This can happen when you are getting +2s or higher, rather than the normal +1s. +2s are given out by VIPs/mods.

Someone with 12 points of rep from 11 scorings (10 +1s, and one +2) would have a ratio of : ((12 - 0) / 11) = (12 / 11) = 1.09.

Summary: achieving VIP status requires that your behaviour on ADISC is regarded as at least 90% positive.