This seems to be a few different questions.
1. How do I take into account holidays?
With teams I encourage them to use their velocity as one of many things to reference when planning the next sprint, not a hard and fast rule. Lets say we have a team of 5 members with a velocity of 20, if one of the team are off for the next sprint I would suggest aiming for around 16 points. Of course, this could be much higher or lower depending on the individual pieces of work, and the team circumstances.
2. How do I plan for unexpected interruptions?
This is where the velocity is really useful. If you find the team are delivery as much as expected and everyone is happy with the output you don't really need to worry about these interruptions - they are absorbed into your velocity and vaguely consistent between sprints. On the other hand, if the teams velocity is dropping (or has a lot of variation caused by interruptions), the customer is not happy with the team output, or the team are not happy with the amount of interruptions, it sounds like you need to focus on reducing these interruptions. Something I have done in the past is create a separate 'task board' where team members make a note of all non sprint related work. This creates visibility of all their interruptions, giving you empirical evidence to use to focus on the reducing these. Often, a few people are identified as the main 'time bandits', and having a chat with them explaining why they should come to the Scrum Master rather than developers directly can help with this. Sickness can be identified here, but not really planned around.