Combine everything except the vanilla and pecans in a heavy saucepan over medium heat. Stir with a wooden spoon as you cook, being very careful not to splash. Scrape down the sides of the pan any sugar granules that may get up there.
When the mixture turns translucent, add the pecans and the vanilla. Continue to cook and stir. The mixture will begin to brown slowly. The whole trick to making good pralines--and it is tricky--is to get them off the heat at the right point. The reading you should see on the candy thermometer should be the "soft ball" temperature, about 238 degrees. It will take about 15 to 20 minutes.
With a large spoon, drop some of the praline mixture onto a cookie sheet, parchment paper, or a marble slab, making discs about two inches across. Allow to cool. Remove with a very thin spatula and wrap tightly.
