Size cutoffs for "biglaw" are pretty arbitrary. Sometimes it refers to the AmLaw 200, sometimes the Vault firms. I've also heard it used as a proxy for salary--i.e., firms paying market rate. Most of these firms do serve large corporate clients.
Vault certainly isn't gospel, but many treat it as such. It's a pretty good indicator of prestige--especially the regional rankings--but quality is such a subjective term. Vault probably reflects the tangibles (assoc. salary/bonus, profits per partner, client size) pretty well, but it all depends on what you're looking for in a firm. Personally, I'm concerned with liking the people I work with, and being trusted with real responsibility, than any rankings.
Also, if you have a specialty interest, Chambers and Partners rankings are probably the best indicator of firm quality in that area.