If you are converting mutual funds at Vanguard, I would guess you would have to submit the transaction by 3:59pm Eastern time on Monday December 31st. It does appear that the markets are open Monday for a full day.
If you're cutting things that close, I would try to do the transaction online, and I would be aware of things like my web session timing out, the fact that Vanguard's website may be a little busy, and that the clock on my PC may not match Vanguard's clock.