In short, assuming you're planning on applying for training contracts at large commercial firms, they are very important.
First off - how do you find them - google is your friend and a quick search brought this up as a starter:
https://www.chambersstudent.co.uk/law-firms/vacation-schemes/vacation-scheme-deadlinesWhat I did when I was applying for vac schemes back in the day was to start with the UK "largest 100 law firms" list and just worked my way through checking their websites to see details of the scheme, getting a rough order in place and then cracking on with the process.
Why are they important?
law firms will expecting to see legal work experience when you apply for TCs. You're still at uni so you can't gain experience via a full time paralegalling job, so you need holiday work/work experience. That's where vac schemes come in and it's the only way you'll be able to get work experience at the large law firms during your studies. Without a selection of legal work experience on your CV you will be at a *serious* disadvantage in obtaining a training contract compared to your peers. ie - unlikely to make it through to interview stage at all. If you don't obtain legal work experience during your studies, then you 1) have less of it to begin with, and 2) have to obtain it after instead, typically via paralegalling for a couple of years.
Secondly - the process of applying for vac schemes, including interviews etc.. is very good practice for TC application & interviews themselves. I remember just how much I improved over the process. If you haven't practiced interviews, you won't perform anywhere near your best in them, and there's no better practice than doing them. Many law firms recruit directly from their vac schemes too so you're missing opportunities in the cycle to obtain a TC too.
On money... it's a problem if you don't live within striking distance of a major city - if you look at the url above some law firms do pay you for the scheme which can offset living costs. Otherwise, it's a case of making the best you can of it, any friends you could stay with for 1-2 weeks? a bunk in a hostel can be had for a week in London for around £180. None of this is ideal but they are options. Any money you do have would be far better spent facilitating attendance at vac schemes than doing a masters - which has little to no benefit for TC applications.
On finding the time... Yes it can be tough but you have to find a way. I didn't study at Oxbridge so can't directly appreciate the pressures you're under, but the honest answer is you have to make the time - assess how you're spending your day and be brutally honest with yourself. Does your academic work truly take up 80+ hours a week such that there's no time for applications? The application process does speed up significantly after the first few as you build up a good answer bank and become more familiar with the process.