Time to choose: Baerwald, Lofgren, Stevenson ?

Sampsa (and Fleib), 
Just a few things:
While it does appear from visual inspection that the headshell of the RS-A1 might rotate in order to maintain tangency to the groove, this actually does not happen, because the wires from the arm wand to the cartridge are stiff enough essentially to prevent it from happening.  Moreover, the English translation of the designer's white paper suggests that he never intended the headshell to rotate. Rather, he is seeking to decouple the headshell from the arm wand.  (This is claimed to be so beneficial that the headshell per se is available as a separate product, which could turn any conventional tonearm with a straight pipe and interchangeable headshell mount into an underhung tonearm.)

With the Viv Rigid Float, if you choose the 7-inch version, then indeed the tracking angle error at the extremes will be about 10 degrees.  However, if you choose the 9-inch or 13-inch versions, the max error goes down commensurately.  While we were in Tokyo, and I was wrestling with the decision to buy one or not, my back of the envelope calculation was for about 5-7 degrees with the 9-inch version (the one I would buy) and less than 5 degrees for the 13-inch version (which I would not buy because I have no turntables that could mount it at the recommended distance from the spindle, and because I think the 13-inch arm wand raises other issues of effective mass and resonance).  It's obvious that the inventor of the arm has run into objections related to the tracking angle error with the 7-inch version (the original sole specification) which led him to create the 9- and 13-inch options.  I did not buy one, but the temptation will not go away, because our son lives in Tokyo, and the tonearm (all 3 versions) is on display at Yodibashi Camera in Akihabara.

Lewm:

I've been looking for that RS-A1 white paper. Do you have a link to it?

Sampsa,
Somewhere I have or had a hard copy.  But I will also check my other computer to see if I have an e-copy.  It's one of those almost laughable translations from the Japanese, but one can make sense of the main points.  I see this problem with my son.  He thinks and dreams in Japanese. When he speaks to a Japanese person, he does not stumble for the correct words. He is a real scholar in this area. But if you ask him to translate a document from Japanese to English, I can tell that there is effort required, and it takes time.  In the RS-A1 white paper, you would see one claim that does not hold up; the paper states that "side force" (skating) is eliminated by the design, when of course it is not; it is only eliminated at the point of tangency to the groove.

Can you, in turn, point me to a review in which anyone compared two different versions of the Viv, so as to be able to say the 7-inch one was preferable?  I did not find that review in my search.

Fleib, Can you explain to me how the RS-A1 tonearm reduces or eliminates torsional effects on the cantilever in the vertical plane? I guess you are referring to its other structural oddity, which is that the unipivot bearing is above the plane in which the headshell operates.

Lewm:
I recall seeing that white paper online, but have not been able to find it again. If you have a PDF, I'd appreciate it.

The RS-A1 unipivot it supposed to be positioned so that the cantilever and tonearm are parallel or ideally on the same straight line. Since not all cartridges have the same angle for the cantilever (I think they range from ~15 to ~22), this will be an approximation. The limited design info I've found doesn't mention torsion, but speaks of "downforce":
http://www.eifl.co.jp/index/export/rs-a1.html

When we usually talk about skating force, we assume that the cantilever is tangent to the groove and pulled along this tangent (only true at those null points naturally). Since the resulting force is at an angle to the line going from the stylus to the pivot, a sideways force results. If the cantilever is not tangent to the groove, the friction force would still be tangent to the groove and thus straight arms would also have a sideways force proportional to the angle between the stylus and the groove tangent.

I've seen reviews of the Viv Lab in The Ear, Stereo Times, and Audio Beat. All reviewed the 7" version and none of them complained about distortion. Audio Beat said they also had the 9" version, but just mentioned that the 7" and 9" sound different "and not in the ways you might predict". Also Audio Beat mentions that the order was the opposite you mentioned: The designer started with the very long version to reduce tracking distortion and then realized that the shorter ones sound better.

Fleib:
I'd appreciate links to those comparisons of Viv Lab versions too.