You really need to stir for a long time to get the condensed milk and vinigar mixed properly. Do not be alarmed. Stirring this recipe for a long time will not make it thin. Leave set for a few minutes and you will be able to turn the bowl upside down and none of it will pour out.
Please note, the amount of sauce provided in this recipe will likely last two servings at most (if used properly). I find that the sauce can be made in larger batches and survives an unnaturally long time in the fridge.