Working on large plate to catch juice, cut lemons in half lengthwise, then very thinly crosswise. Discard seeds. Pack enough lemons and any juice to measure 2 1/2 cups. Transfer to large nonreactive pot. Add 5 cups water; bring to boil. Reduce heat to medium; simmer 10 minutes. Remove from heat; let stand uncovered overnight.
Measure lemon mixture (there should be about 5 1/2 cups). Return to same pot. Add equal amount of sugar (about 5 1/2 cups). Scrape in seeds from vanilla bean; add bean. Add pinch of salt. Bring to boil, stirring until sugar dissolves. Attach clip-on candy thermometer. Maintaining active boil and adjusting heat to prevent boiling over, cook until temperature reaches about 223˙or 224˙is fine (see below) , stirring occasionally, about 30 minutes. Cool to room temperature. Transfer to jars. Cover and chill. (Can be made 2 weeks ahead. Keep refrigerated. Bring to room temperature before serving.) Makes about 7 cups.
If you don't want the marmalade tooth-achingly sweet and liable to crystalize in the fridge, you can also add a little bit of Pomona's pectin by reserving a cup or two of the sugar as you begin to boil the mixture. As it gets to about 215˙ (where there will be a plateau), add in the calcium mixture, then, mix the Pomona's into 1.5 c of sugar and working quickly, but not all at once (this is an important point), add the sugar/ pectin mixture while stirring vigorously to prevent clumping.