Hi @Tu’ssant Rantanen and congratulations on making this a goal at your age.
I like to underwrite my vacancy at 5% although it's really been closer to 3%. This will be very market and management dependent. My market doesn't have much for affordable housing so I have no problem filling them and I get leases to start back to back to keep vacancy low. You will need to account for some vacancy as you do updates which is why I have averaged a little less than 3%.
For maintenance, it will be property, and even a little bit tenant dependent. I have properties that don't have much maintenance, and other that feel like a money pit. Some tenants can't do anything themselves and want everything fixed, others I have never contact me for anything. In general, I underwrite between 6-10% depending on the age and condition of the property.
Hope this helps and remember, your first one is about learning and making a little money. You don't need a home run on your first deal.