Back when I recorded bands who wanted to cram an entire album into 2 days, we always found ourselves starting vocals after spending 20 hours (on Saturday and Sunday) on recording drums, bass, and guitar overdubs. Never were these singers ready to sing. In fact, they had been sitting around extremely bored the entire time. To ask a singer to give me 100% after what was basically a 20 hour car ride, is just stupid. There is no way you could get the emotional impact or even work ethic required for a great vocal take..
Your record is going to be on your mantle for years and years and years. Do you really want friends and family members to ask you to play it 10 years from now and you feel embarrassed because you weren't 100% that one day you recorded it? I think that's a terrible way to live. Unless you are under some sort of deadline, screw it! Just reschedule. Your record is important! If it's not important, why did you bother making it in the first place?
If you have serious plans of marketing your record, how do you expect to sell any copies if the vocals aren't money? Besides, to launch a record properly is an enormous amount of work. Compared to all the work it takes to seriously push an album, the vocals are a small percentage of that (even if they one of the most important). So if your voice is hurting you, go back to the marketing machine. You can still put a full day of work in for the band. This way you space it out so your voice has time to rest.