Combine the sugar, brown sugar, condensed milk, water, corn syrup, salt, and lemon juice in a heavy saucepan over medium heat. With a wooden spoon, stir until the sugar mixture is homogenous and begins to simmer around the edges of the pan. Add the butter and stir until incorporated.

Using a clean pastry brush dipped in warm water, brush down the sides of the pan as needed so that no crystals form around the edges. Add a candy thermometer to the pan and cook, swirling the pan occasionally, until the mixture reaches the soft ball stage on a candy thermometer, 236° to 240ºF.  (That is, when a bit dropped into cold water forms a soft ball that flattens.) Add the vanilla and pecans to the pan, remove it from the heat, and stir the mixture until it has cooled slightly and the sugary syrup becomes sliglty less glossy and a bit opaque, usually 2 to 3 minutes.

Working quickly, drop by the spoonful onto parchement paper or buttered aluminum foil. The pralines will set up as they cool, usually within 30 minutes.

Remove from the paper with a thin knife.   Pralines can be stored in an airtight container at room temperature for up to 2 weeks.  

Cook's notes: 

The water isn't absolutely necessary, but since you are blending four different types of sugar here, it does help form a cohesive mixture before the sugars begin melting and cooking. The addition of a few drops of lemon juice discourages crystallization while the mixture is cooking to the soft ball stage.

Make sure to have your parchment paper out on your counter and have a clean spoon handy so that when you are ready to spoon the pralines out, you can do so quickly. Once the mixture begins to lose its gloss and becomes more opaque, you will need to work quickly since it will continue to firm up in the pot if you take too long.

Conversely, if you do not stir the mixture long enough after removing it from the heat, the pralines will be more gooey and caramel-like and less crisp/grainy. This is the trickiest part -- knowing how long to stir and when to begin spooning onto the parchment. With practice you will begin to see the slighty nuanced changes in the sugar syrup that let you know when it's time to spoon the pralines out onto the paper. Good luck!