At least in Germany this would either not be a good deal for the employee or employer.
Also I'm not sure how legal it would be in Germany and other EU countries.
Edit: thanks, spelling
If you could sell your future interest in refundable vacation days now, for more than it cost you to buy them, that would be an arbitrage.
Pretty neat, but its capped, otherwise I suspect there would be people who elect to work for 4 months a year (or something similar) for 1/3 of their salary.
In the Netherlands there's even a law that states that you can request working part time in basically any fraction of a full work week and the company _must_ grant it unless they can show grave impact to their business that couldn't be resolved by hiring and/or training more people. Your pay is altered in the same proportion, ie you can work 50% of the time for 50% of the pay. This would probably be more difficult in the states, since health insurance is not as strongly linked to having a job over here.