How would they work?

The point I made before though is that you're dealing with natural limits, rather than a limitation of labor power. We have tons of labor power and machinery to amplify it, such that the cost of say a plastic controller is cheap as shit if we're just looking at labor costs and the cost of extracting materials. Yet, if we went about supply controllers and other doodads to all 8 billion or so people on the Earth, we're going to have resource problems. So, you have a resource demand that quite a few people want, that we can't resolve with the law of value.

I think the LV would work well in conditions where natural resource limits are not a major issue, and the vast quantity of labor time is spent on unskilled labor requiring little in the way of education. Those were the conditions in 19th century Europe and for much of the early 20th century. Even today, that still is very much the case in a lot of ways, but increasingly skilled labor and education matter more to the production process, and LTV doesn't have a good way to really calculate how much worth skilled labor has over unskilled (this is a pretty big bugbear - I agree with Marx that it is theoretically possible to reduce skilled labor to unskilled, but if you try to build a system around it you're going to find a lot of practical problems calculating just how much a skill is worth, and a lot of people who think their skills are worth more, and you've just created for yourself another calculation problem). It seems good and decent for a factory worker to be paid compensation in labor-hours, but it's harder to calculate for an engineer or a teacher. (Of course, money too is an inadequate tool for calculation purposes for much the same reason - and that is where I really think capitalism is going to run into the ground, when the class of skilled and educated workers is sick of their education being constantly devalued.)

So yes, where do we go from here? We've come to the conclusion that vouchers really don't help us with the calculation problem, so their primary purpose is to solidify the social contract we enter when we decide to work for the socialist system - if you do X amount of work, you are entitled to Y in return for your work. Yet, we see a situation where workers cannot get Y, or they think the calculation for X is unjust and arbitrary. (Just imagine a system when the overseer of labor arbitrarily decides to give all their friends "exemplary" marks, and the people they don't like "casual" marks for the quality of their labor.) In pushing the labor voucher system as THE SOLUTION, in the face of these issues, you are maintaining a blindness towards the nuts and bolts of how a planned system would need to function if it wants any sort of stability. Clearly you need something more, a sense that people at the local level - individually, and the whole communities to which they belong - are really part of the system, rather than subjects of the central bureaucracy. This goes beyond just telling people they have a vote and calling it good now that you've replaced wage-labor with… wage-labor.

Unicorns and rainbows, oh my.
This is the kind of retardation that lets right-wingers ridicule us so easily.

Nobody said that labour vouchers are a 1:1 accounting of labour, you're totally misinterpreting it. Obviously you wouldn't get back exactly the productive value of the labour you contributed, that would be impossible to calculate anyway, some people will get more back than they 'deserve' and others will get less even if everyone receives the same amount, but that's not the point. Labour vouchers aren't just a straight replacement for money, they're a safeguard against someone ordering 10,000 yachts for themselves. I think labour vouchers should only have a small variability from person to person if any, and it shouldn't be incentivised to spend them all anyway if you don't need to.

So sum up, even if it takes 1 hour of labour to produce a plastic widget, it would be priced far higher if there's some kind of unsustainability involved in its production.

What is your big issue with that? We have the technology for near-perfect recycling, just not the political will to put it into action because it's cheaper to just destroy and excavate more resources.

Labor vouchers don't create the issue of resource limitations, so I don't see how this is an argument specifically against labor vouchers and for something else which would be better in regard to this issue.

You talk as if this is hypothetical, but many countries do have different sales tax or VAT for different items, and this type of regulation has existed for decades.

That doesn't sound so much like a calculation problem, but a social one; and, like with the issue of limited natural resources, it is not something new that comes into existence with labor vouchers. Cockshott and Cottrell propose that training time necessary for working in production of a particular item should be priced in, but that's the price of the thing, not the salary. In their proposal, the public funds education and job training, as such you have no right to get extra compensation for having education. Though realistically, unless the whole world is under that system, we'll probably have to compromise on that.

Speaking of being realistic, I don't see any realistic alternative to labor vouchers.