Substitute for vanilla caster sugar I'm planning on making a lemon tart and the recipe calls for "100 g vanilla caster sugar". Over here (Netherlands) vanilla caster sugar isn't really sold in supermarkets, so I'm looking for a substitute. 

One thing I do have acces to, is sachets of vanilla sugar. I have a feeling I can't just use those however, since usually only one sachet(9 grams) is used in a recipe. I'd have to use 11 of those sachets to get the same amount of grams, which would probably make it taste way too vanilla-y. 

Should I just use regular caster sugar instead and add some vanilla extract, or maybe regular caster sugar and a singe sachet of vanilla 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.