Preheat the oven to 160c.
In a medium bowl, combine the flour, sugar and salt. Chop the vegan butter into cubes and add to the bowl with the flour. Work the butter into the flour by pinching everything together and rubbing the ingredients between your finger tips. Do this until you have a bowl of sandy textured flour with no large lumps of butter. The mixture will feel quite dry. Turn the mixture out onto a surface and knead it together to form a large ball of dough.
Line a 23x33cm rectangular cake tin with a sheet of baking parchment. Press the dough into the bottom of the tin, forming a smooth, even layer of dough. Prick the dough lightly with a fork all over and place in the preheated oven to bake for 20 minutes or until very lightly golden on top. Remove from the oven and place on a wire cooling rack. Allow to cool fully in the tin before transferring to the fridge.
Meanwhile, make the caramel. In a large saucepan add all the caramel ingredients except for the vanilla and the salt. Place over a medium/low heat and stir with a heat-proof spatula until all the butter is melted and the sugars dissolved completely. Using a probe thermometer, bring the caramel to 115c (you may have to increase the heat to get it there, but trust me, it will get there!). Remove from the heat, add the vanilla and salt and stir continuously (off the heat!) for around 10 minutes while the caramel cools. It will thicken dramatically!
Once thick, set aside and allow to cool for a further 5-10 minutes (it should still be runny enough to pour or spoon over the shortbread, so keep checking the texture regularly). Pour or spoon over the shortbread in the tin, even out with your spoon and then return to the fridge for at least 15 minutes.