I actually have the same opinion as you on "unlimited vacation". I'd rather know what the company explicitly allows and use it guilt-free than have to ask myself every time I take time off, "Wait, I mean, I know I'm technically allowed to do this, but am I
allowed?" People who go in to companies like that guns blazing, oblivious to social matters like colleague perception, are usually destroyed pretty quickly.
So, I can definitely see where your concern arises. I don't want to make it seem like you should do any quantity work that the employer throws your way. There should be a basically agreed-upon range of value delivered in person-week units; the employer should expect to get some value within that range per week, and the employee should expect to provide it on an ongoing basis. This value range is something that is going to be negotiated between employee and employer and the only way to get a realistic feel for it is to undertake a relationship and see where you land. By accepting the standard timeframe for value evaluation to week-units, it matters much less whether you were in your chair from 2pm-4pm all 5 days of the work week. It just matters that you got your work done.
I don't have an exact way to quantify it, because again, in knowledge work, employee productivity isn't really calculable by a simple equation. It's really just about whether the company feels adequate progress is being made and whether the employee feels that his work-life balance isn't falling apart. As long as both of these things are synchronized on an individual basis, there is no problem and no hard rule.
It's hard to get to this because as you said, an employer feels that they're not getting their money's worth if there isn't 40 hours on the clock. However, that's based on the antiquated model of production where time-at-station pulling levers was directly correlated to the value provided to the company. In a factory / assembly line context, that approach still makes sense. For knowledge workers who depend on creativity, like software developers, it doesn't -- it's not an accurate way to measure performance. The challenge for us is in educating employers to view knowledge workers appropriately.
On HN we're all probably familiar with PG's Maker's Schedule essay. I believe that schedule arises naturally because our brain is trying to self-optimize and do work in the most productivity time period, which is usually overnight when there are minimal interruptions. We should all be free to engage with that. It will result in both happier employees and superior work product.
Finally, I think there's a more basic component at play here that I don't actually think we'll overcome anytime soon. The workweek is entrenched into American lifestyle now. People are taught to expect a 9-5 and to be suspicious of those that don't have one. People are taught that colleagues should all go hang out in a big office building for 1/3rd of the day 5 days per week. This is a common expectation even if it's unsuited for modern work. Most people, even most knowledge workers, like this arrangement and don't want it to change. The social costs of doing something unusual with your work day can be substantial, especially if you're in the midst of a period of financial difficulty. I don't think that will change any time soon and I don't know that it'd be a net social good if it did.
However, there are companies that accommodate people that want to work differently. They can be found with some degree of discretion and specific searching.