Substitute for vanilla caster sugar
The answer depends on what happens to the sugar in the recipe. 

If the sugar is used as a flavoring/sweetner inside the tart  filling (i.e. not sprinkled over the top or in the crust), you can do a number of things - you can take some caster sugar and vanilla extract, as you suggest. This will add some moisture and dissolve the sugar a little, but if this is in the filling this shouldn't matter. You could also add some vanilla seeds (the scrapings from inside a bean).

If it is used in the crust or as a topping, I would get some vanilla bean and scrape one into a volume of caster sugar. Alton Brown recommends 1 bean per 2 cups of sugar.