I have entered vacation days for all people involved but I do not see them in gant chart? what do I need to account for vacation days to see impact on milestones?
Vacations (days off in resource properties) show up as yellow bars on the chart. They do not have any other meaning except for yellow color, sorry.
Holidays which are defined in the project calendar show up as very thin rose bars. They do have impact on task lengths automatically, which may or may not affect your milestones depending on other conditions.