I'll add in my perspective as well. I do agree with most of the information contained in this thread, but some of the information seems a bit misplaced.
First, the Vault survey is not the best way to go about choosing a firm. This has been said. Unlike even U.S. News, the information that produces the Vault survey is entirely based on responses from practicing attorneys. In a way, this could produce an accurate result, but my experience suggests that the majority of partners and associates don't respond to the survey and thus, the low response rate does it in. Furthermore, the survey doesn't allow respondents to rank firms, you simply attach a numerical value (from 1 to 10) based on the prestige of the firm. When you take the survey you cycle through screen after screen of firms (we are talking 100+ firms) and by the time you assign a value to the last firm you've probably forgotten what you've assigned to 70% of the other firms. You don't have to assign a value if you don't know the firm. Thus, what you get is associates assigning values based on previous year's results. For example, "I know Wachtell is the #1 vault firm from last year. I have never worked on a case/deal with Wachtell, but because they have such high PPP and they were #1 last year, I am going to mark them as a 10 for prestige." I do agree that it is a general yardstick for firm prestige, but what most law students should be focusing on is practice areas (if you know what you want to do).
The Chambers guide (the actual guide as opposed to the chambers associates guide/website) is the best resource out there. As others have stated, you can see which firms are actually the best at what they do. So Firm X may have a band 1 ranked M&A practice, but not be ranked in Real Estate. If you really want to do real estate, then it wouldn't make sense to go to Firm X when Firm Y has a much better reputation in that field. This directly plays into exit opportunities (discussed more below). More reputable practices have better clients and better work, which will yield better exit opportunities down the road.
2.For QoL issues, I would turn to the Vault Associate surveys. While not perfect (see low response rate issue above), these surveys are the only measurement of how associates rate their experiences at firms. My experience is that firms encourage their associates to fill out these surveys. A high response rate should yield a relatively accurate picture of firm life.
Also, the Chambers Associates guide is decent here (along with ABL's/Lateral Link's career center). Note that these guides are produced by interviewing existing associates at the various firms. The companies then aggregate data and decide on a "story" to tell about the various firms. They may not be wholly representative.
QoL at bigfirms is less about pure number of hours worked and more about how much an associate can control her/his work. For example, junior associates will undoubtedly work late nights and on weekends and thus, the question to answer is whether the firm allows/encourages associates to work from home on weekends or late nights. Many firms don't care where you work so long as you get your work done, however, other firms expect juniors to be in the office when they work. This is difficult to determine, especially as a law school student, but reaching out/networking with current junior associates is important. One caveat is that this is more of a practice group/partner issue and not generally consistent on a firm wide basis.
3. Exit opportunities. In order for exit opportunities to be an issue, most associates need 3-5 years of experience. Few if any associates that have less than 4-5 years of experience will be lateraling in-house at a client. It is more likely that that associate will lateral to a non-client of the firm. Clients hire from law firms when they have experience working with an associate and have confidence in/trust that associate. Generally, the V10s-20s will offer superior exit opportunities to V30s and above in so far as the opportunities themselves will be better. You are more likely to land at a Fortune 500 company or large bank than if you are at a more prestige firm. The reason is that you've worked on deals/matters that routinely occur at these companies and banks so the theory is that you are more likely to hit the ground running than if you worked at a less prestige firm that did more commodity type work. The other reason is that the people who are in charge of hiring at these places likely worked at the top firms and therefore, they want the "best" people working under them. This may or may not be true, but it is the way that lateral hiring occurs.