Upvote:3
Vienna is a popular tourist destination, with many hostels. You will be able to find the beds(probably in different rooms), if you leave it till the last minute; or you might not. Popular hostels like St Christopher's Inn get booked out pretty soon.
It is best to book in advance, I would say. Having said that, it is never advisable to book way in advance, as the rates seem to be higher then (Personal Experience). Best to book it a week or 10 days before. There are many booking sites - www.hostelbookers.com is a good one. However, it is better if you book directly with the hostel (you will most probably get the cheapest rates).
When you book in through a booking site, a 10% deposit is required. If you book it directly with the hostel, you might not be required to pay the deposit.