The different cuts from Vandoren are basically an exercise in changing the response and tone colour of a reed. Simply put, a viable reed has to conform within certain basic parameters in order to vibrate well, i.e.to have good tone, intonation and response throughout the given instrument.
By response, I mean the immediacy or amount of resistance of a reed to the breath, and also the flexibility of the reed once it is vibrating.
By tone colour, I mean the relative amplitude (i.e. what you hear most of) of the high upper partials.
I reckon I sound exactly the same on pretty much any reed, but the subjective ease of producing what I want can vary enormously from one type of reed to another. E.G. You will almost never catch me using Vandoren Blue Box on tenor, but I can happily play them on soprano and clarinet.
Theoretically, if you had all the Vandoren reeds in the same medium strength (2.5/3):
Java Green have the least resistance and the most flexibility of the Jazz/Rock orientated reeds and will most easily produce a bright tone with some warmth and a lot of very high upper partials which give a sort of 'hash' to the tone. Typically this is a more Vintage sound, with good attack and projection, but less meat in the centre of the sound. I grew up with these and am sort-of addicted to them, although they can be a little hard to handle on mouthpieces with high baffles.
Java Red are an attempt to lower the predominant upper partials and to provide more meat in the tone and a touch more resistance in comparison to Java Green. Consequently they are a little less flexible. I think they are a more modern tone, but feel as if they lack some of the warmth of the Greens. Not my personal choice.
ZZ are, to me, more like a Java with less very high frequencies and a little more aggression. My next favourite to Java Green because they retain good flexibility and response. These work out about the same resistance as Java Green.
V16 are even less flexible and have the ability to cope with high-baffle mouthpieces better than Java Greens. They have the best projection of any Vandoren reed, but are solidly geared to a bright and aggressive tone. They are quite a bit more resistant (quarter strength harder?) than Javas or ZZs.
Blue Box are the original Vandoren Classical reed and produce a very fast response and a very clean sound (little or no hash) with very balanced upper partials and loads of heart, but they are particularly designed to work with French classical mouthpieces that have, in the main, low baffles. They are the clarinettist's and the classical sax player's gold standard.
My experience is that although some players like them with jazz-type mouthpieces (Link, Larsen, Meyer etc etc), I find that these reeds are inclined to sound hollow, and IMHO, unconvincing unless used on close classical mouthpieces. If you use a Jazz or Rock type mouthpiece that is at all asymetrical in the facing or baffle (i.e. duff) steer clear of Blue Box because they tend to generate squeaks and squeals very easily because of the very thin and responsive tip. They are a half strength harder than Javas.
V12 Are Vandoren's attempt to provide a Classical reed with even more tonal depth and thickness and somewhat less very high upper partials, but the sound is still very well balanced. I like these on clarinet because they produce a smooth and opulent tone more easily than Blue Box but I have never really liked them on sax because the bigger reeds are perhaps less responsive.
Vandoren advertise these as a little softer than Blue Box, and on Clarinet that is true, but strangely not so on Saxophone. To me, they are quite a bit harder (quarter strength harder than Blue Box, so 3/4s harder than Javas?).
V21 are designed as a 'Universal' reed with less immediate refinement than Blue Box, better response and clarity than V12, more heart and solidity than the Javas, ZZs or V16s. Vandoren claim that the V21 also copes with wide interval leaps very well.
I really like these reeds on soprano sax and clarinet. where I tend to use closer mouthpieces for increased stamina. They last well and IMHO have the best balance of sound of any of the classical reeds. I have yet to get into them on any other instrument. They are, like Blue Box, pretty much a half strength harder than Javas.
I hope this helps people to choose what might suit them best.