It's an interesting point. Where the timeout period is greater than zero, then players who are not on vacation can compete on a level playing field with those who are using their vacation either to be away from the board or to manage their games.
However, when the timeout period is zero, and only a timebank is used, the vacation period does indeed become a substantial advantage because part of winning a hardcore tournament is the ability to make moves very frequently.
The quicker you move in a hardcore tournament, the greater an edge you gain over your opponent. This edge doesn't exist in other other tournament formats where as long as you move within the timeout period, your opponent cannot gain an advantage over you by using vacation.
Something we need to think about.
A simple solution, as mentioned, may be to remove vacation immunity from hardcore tournaments. After all, if you enter a hardcore tournament you know how long you will need to make your moves so as long as you enter long before you intend to go on vacation it shouldn't be a problem. And if the tournament doesn't start in time, then you can always withdraw.