Using an electric mixer or a stand mixer (with the whisk attachment), beat on medium-high speed aquafaba and cream of tartar until the mixture becomes foamy and the mixture becomes opaque. Continue beating until reach medium peaks.
Place water and sugar in a medium saucepan. Stir to combine and dissolve the sugar. Insert a candy thermometer into the sauce and heat over medium heat.
Simmer the water and sugar mixture until it reaches 240º F (120º C) on a candy thermometer. Remove the sugar mix from the heat once it reaches that temperature.
Turn mixer to low speed. Very slowly and carefully pour the sugar syrup into the fluff aquafaba in a thin, steady stream. Try to pour the stream right between the whisk and the sides of the bowl, so the syrup doesn't get splashed by the whisk.
Once all of the syrup is in, set mixer to medium-high speed and continue whipping. The mixture will deflate at first, but then it will thicken and fluff up.
Continue to whip for 8 – 10 minutes, or until the mixture is thick and glossy and the base of your mixing bowl is no longer warm.
Stir in vanilla and continue whipping until the fluff has cooled, about 2 – 3 more minutes.
Use immediately or pour into an airtight container and store in the refrigerator for up to a week.