When I upgraded my membership in November, 2009, premium members received the following "bump" in their vacation:
Gold: 1 extra day per month if renewed monthly; 14 extra days if renewed yearly.
Diamond/Platinum: 2 extra days per month if renewed monthly; 28 extra days if renewed yearly.
The above is in addition to the 2 days per month that everyone receives. I don't think that this policy has changed.
Premium members get at least 30 days vacation per year - so this does allow for quite a few 'unable to get back in time' episodes - even when there is a one day minimum.
Doesn't vacation accrue at the same rate for everyone (24 days per year)? It's just that unused vacation can be stockpiled in greater amounts (60 days and 90 days as maximums) by the higher-ups.