The wood that's devoid of leaves is likely dead or so weakened that it will not re-sprout well if it hasn't done so by now. While they are not pest- or disease-prone plants, it's possible that roots were lost or the main stem(s) was killed due to factors like drought, overwatering or poor drainage, cold snaps, injury from cuts into the bark, or girdling by vole gnawing at or just below soil/mulch level. Wood-boring beetles can also target various woody plants undergoing stress due to drought or excessive soil moisture. If the plant is sprouting from its roots, that is an encouraging sign that enough healthy roots remain that will allow it to at least regenerate from root suckers.

It is difficult to guess as to the original issue without photos to assess, though it's still possible that no outward symptoms are yet apparent that point to a particular cause. Since you've had the plant for a number of years, maybe conditions changed last year that resulted in the dieback, or there was an unusual weather event, like sudden cold after a warm spell broke it out of dormancy. In prior seasons, was the Wisteria usually checked for water during dry spells? Or is there a source of runoff nearby that floods its root area, such as from a downspout, leaky gutter, or natural gulley in the landscape? Is there lawn adjacent to its planting site that is mown or edged in such a way that equipment accidentally hit the bark? Was any herbicide used for weed control in that area?

With luck, the new growth will begin to regain lost ground and fill back in. For now, all you can do is to cut back all dead wood and attach the new vines to the support if they need help reaching it at first.

Miri

Photos of the overall plant and its location would help, plus close-ups of any damage to the main stem(s) that might be visible - cracks, scars, sunken areas, sawdust, or holes - though it's certainly possible no outward symptoms are present. Unfortunately, root rot would only be determined by uprooting the plant, which if it's coming back from some root sprouts, isn't ideal since presumably you'd like to preserve that regrowth.

If the collected humidifier water was at least a gallon's worth each time, that should have been helpful if the weather was too dry. Less water wouldn't have necessarily hurt, but may not have prevented any notable level of drought stress.

Once dead wood was removed you could try splitting-open the lower portions of stem to see if any wood-boring insects or staining from disease is evident. Some types of either would appear just beneath the bark, while others would enter the heartwood in the stem's center. If no damage is seen, the issue is likely root-related, though differentiating between overly-wet and overly-dry causes of root death can be challenging once enough time has passed; vole feeding tends to be more blatant damage, at least.

Miri

Miri,

Here are some of the photos you requested.  A couple of more are on the way.

John

Miri,

Here are the rest.  Thanks for taking a look at them.  Please let me know if you need anything more.

John 

Apologies for the delay, and thank you for the photos John. The stems look dead, unfortunately, and should be cut back to the ground or to any point where foliage is emerging, but it sounds like that was only from root sprouts to the side of the original stems. As to why they died back, we cannot narrow-down the possibilities much from its current condition, but root dieback due to overly-dry or overly-wet conditions is likely, simply because that is most often the case with landscape plant losses. Pest insects or wood-infecting pathogens are possible, but often they are secondary issues that arise after an environmental event (drought, flooding, etc.) first causes the plant stress and makes it a more vulnerable target.

One other possibility is scale on the bark of the main stem near the ground - your last photo - though we cannot see it clearly enough to be sure. Are you able to examine and photograph that lower stem portion? We're looking for white or gray (frustratingly, essentially the same color as the bark itself) circular or oval lumps that have glued themselves to the bark, though you can usually dislodge them with a fingernail. (Normal lumps on bark, called lenticels, won't flake off. See link below for some scale image examples.) If present in high enough numbers, some species can cause plant dieback as they suck sap (or plant cell contents) for nourishment. In the event scale are present, since the stems have already died, cutting back old infested growth would still be the recommended action to take. The regrowth can then be monitored and sprayed with organic or other minimal-toxicity pesticides to suppress them until beneficial insects can help reduce their numbers.
https://extension.umd.edu/resource/armored-hard-scale-trees-and-shrubs

Is the Wisteria in at least 4-6 hours of direct sun in summer? (Six or more is ideal, but they are a bit flexible.) If not, it will grow more slowly and probably flower less abundantly, as you have noticed. The root competition from the neighboring Nandina shrubs may also be slowing down its progress (both original growth and recovery).

Miri

Miri,

Here's the photograph of the lower stem you requested.  I wasn't able to scrape off the little white/gray markings with my fingernail.

John

Thank you for the photo. While it's not quite in focus, the pale specks seem to simply be lenticels, which are normal pores in bark that on some species can be fairly prominent lumps of a slightly different color than the rest of the bark. You description of them not being removeable supports this.

Perhaps wood-borers, some of which can be beetles only a few millimeters long as adults, are in the stem but have not yet emerged. There may be a hole or two visible, but we can't be sure since it's not clear. The emergence holes themselves would only be about a couple of millimeters across. In either case, if present, their damage has been done and they will be excised along with the dead wood being removed.

While this doesn't rule-out root rot, the two issues can co-occur, since often borers are drawn to plants under stress. (They can "sniff-out" the chemicals released, especially in the case of root rot.) Similarly, some canker-causing fungi are more easily able to infect stems on drought-stressed plants.

Miri

I was checking the stem for little holes, and the base of it just broke off in my hand.  I tried to make this photo as focused as possible.  Thought it might lend some clarification as to the cause of the vine's death.

Interesting...well, it's not conclusive unfortunately, but it does suggest decay. (Of course, it could be secondary wood decay after the stem had already died weeks or months ago, rather than a direct mortality cause, so that's alas not as helpful for our investigation.) If you'd like, you can shave-off the thin outer bark layer at various points along that lower stem section and look for small tunnels snaking around in the wood, but such beetle activity also could have been secondary. You could split the stem in half too and look down the center for tunneling.

Does the point below that - the stub left in the ground - look gnawed or chewed in any way around its perimeter? We don't know if moles specifically consume Wisteria or not, but they can cause similar snap-off damage to other woody plants.

Hopefully the regrowth you have from the root system remains vigorous and is able to eventually replenish the vine's intended coverage.

Miri