There's something that seems to be missing from the discussion. Everyone's looking at the expected rate as experience/season. While this is usually the most important rate, it is one of two important rates (and a third factor). An average of 5.75+Aura gives you about the same average as you'll get from tractatus. Noting we can pull off an extra +1 with commentaries and that getting an extra +1 from resonance is fairly easy, getting to Q10 is the lowest typical quality of anyone with Good Teacher (need Com 0) or with Com +3. So it has been established that using vis gives about the same quality as using tractatus, and a worse quality than using a low-level summa (typical quality >15).
So how about the cost? The low-level summas are relatively inexpensive, and you commonly expect to get to roughly a 6 with them (see ArM5 and Covenants). So, assuming you don't have a good mid-level summa, you'll start making the comparison to tractatus after level 6. If you do have a good mid-level summa, we can probably estimate holding off on the comparison to at least level 11. Since the average quality from a tractatus and from vis are the same, all we really need to do is compare the costs. From levels 6 to 15, it's just two or three pawns, which seems about on par with tractatus costs (again, based of Covenants). The tractatus price for a quality 10-11 won't be based on your Arts, but the vis cost will keep rising. So using tractatus gets better and better.
That leaves us with the third factor: availability. First, I'll address vis. It will get harder and harder to get the necessary quantities of a single type of vis, with the exception of Vim vis (you'll create too great a demand within your covenant as well as simply running into regular supply limits). However, with tractatus, you just need to make a deal, which typically won't have quite this issue. Second, I'll address tractatus. Eventually you'll simply run out of tractatus. Then you're just stuck with that route. You could get some lower quality ones at reduced price and higher at increased price, keeping the averages all about the same. But you'll still run out.
So my overall recommendation would be to use vis from level 6 to level 15 or 20 for your favorite few Arts, but only when you can't get a hold of a good mid-level summa. Once the cost starts getting noticeably higher than the cost of equivalent tractatus, then switch over to tractatus. But if you're considering the rest of your Arts, most of which are unlikely to advance past 20, perhaps not even 10, skip vis entirely.
Edit: Note, all this is for an individual. If the cost of a tractatus can be split among several in a covenant, the argument against using vis except early in Arts you want to push beyond what tractatus can handle only become stronger.
Chris
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