OK, one week later and five samples later, I think I've pinned down how I'm going to spin the yarn for the Vine Yoke Cardigan.
Plan A was to try a 3-ply versus a four strand cabled yarn. I spun up a bunch of singles at about 32 wpi. I was trying to spin my default yarn, and spinning from the fold at 7.5:1, that seems to be 30-32 wpi singles. Before a fulling finish, the 3-ply was ~14 wpi - after finishing it plumped up to 10-12 wpi, so right in the range for heavy worsted weight.
The grist on the 3-ply ended up at 1067 ypp - way lighter then the required 840 ypp - but I swatched it anyway to see how it knit up. I did the same type of swatch as I did for Manon, using US 7/8/9 needles (pattern calls for US 9/5.5 mm needles). I liked the fabric I got with US 7s, but the gauge was 20 sts/40 rows per 4 inches*. Gauge with the other two needle sizes was 19/38 and 18/36. I'm aiming for 16 and 32, so this was out. On US 9s there were lots of gaps between the stitches, particularly in stockinette, which would be problematic for the yoke detail. So...the 3-ply is out.
The four strand cable - I used the same singles (~32 wpi), and plied two 2-ply yarns at 9.25:1, aiming to overply quite a bit. Then these two 2-ply yarns were plied together again, in the same direction the singles were spun. The resulting yarn was ~12 wpi before washing, and about 10 wpi after a fulling wash.
Here's a close up of the sample card I made: sample of singles with ratio, 2-ply sample, and the final yarn in washed and unwashed states. I did the same swatching, same needle sizes. Stitch gauge for four inches with US 7/8/9 was 18/17/16 respectively. The fabric that I got with US 9 needles was just about perfect. The garter stitch was squishy and cozy, the small bit of stockinette in the swatch was nicely filled in. All was looking good. And then I figured out the grist - 800 ypp. I've never been this technical about my spinning or tried to match a commercial yarn before, so I don't know if this is as good as it gets or not. Could I get a perfect match? Or is a 40 yards per pound difference close enough? One thing it does mean is that I'll need 1.56 lbs of fiber instead of 1.5. Since I started with 2 lbs and ended up using maybe a quarter of a pound for my sampling, I've got plenty of fiber. But let's move on...
Inspired by my getting as close as I did to the target grist, I went a bit hog wild and decided to do a straight 4-ply with lots of plying twist. Somehow I convinced myself that more plying twist (and consequently less yardage per unit weight) would get me closer to 840 ypp. For those of you playing along at home who may have picked up on my error in logic, you can go ahead an laugh. I dutifully spun up my singles at 12:1 (~30 wpi this time), plied the everliving crap out of them (also at 12:1), and got a 10 wpi yarn before washing. I finished the yarn by soaking in hot water and letting it cool down before squeezing out the excess and letting dry. The finished yarn was also 10 wpi.
Strangely enough, adding more plying twist and thereby getting less yardage for the weight of fiber did not result in getting closer to my magic 840 number. This yarn weighed in at a hefty (aka dense) 700 ypp. Ooops! My swatch bore a striking resemblance to EliCa's Kevlar socks. Stitch gauge was 17/16/16 spi, but the fabric was stiff and the antithesis of drapey. Porpoise-spin-zen fail!
After much searching of the Spin Tech group on Ravelry, I decided to see if I could salvage the 4-ply. I ran it back through the wheel and took out some of the plying twist. The yarn loosened up quite a bit, and after a soak in hot water, I knit (yet) another swatch.
This time I ended up with 17/16.5/16 spi, but the grist was 768 ypp. Going in the right direction, but I didn't think I could take out any more plying twist and have the yarn stay together. So fabric/squish win, but the cabled yarn was still closest.
Emboldened by my success with un-plying the overdone 4-ply, I decided to try overply the original 3-ply I started with to see if I could alter the grist just by changing the plying twist.
After a fulling wash, I ended up with a yarn that was probably about DK weight, came in a 1093 ypp, and probably needed a US 4 or 5 to get a nice fabric. It also bore a striking resemblance to wire. Any spinning masters that might happen to read this are welcome to weigh in on their ideas about what the crap happened here. Just I cannot explain why adding more twist (and presumably shortening the yarn) ended up increasing my ypp.
In any event, I think I have a winner. I really like the way the 3- and 4-ply yarns look knitted up, in terms of how the different purples blend together, but the cable feels that much better knitted up. So that's what I'm aiming for.
Singles spinning has already begun...
* I admit that I didn't actually measure row gauge. One of the very nice things about garter stitch, however, is that the row gauge is twice the stitch gauge. I may not be very good at math, but multiplying by 2 I think I can handle.