I have a recipe for an apple-strawberry crumble that calls for "1 vanilla sugar." I don't have any, so I'm looking for advice on how to substitute vanilla and sugar for the vanilla sugar - as well as estimates on appropriate amount, since the recipe left that out! For estimating guidance, it's supposed to be a 9x13 pan.
My best guess would be what Stephie already hinted at: "1 vanilla sugar" means "one sachet of vanilla sugar". I have seen these in different European countries, and they normally contain the "standard" amount of synthetic vanillin to flavor a recipe of up to 500 g of flour, and just a little bit of sugar, not to make it sweet, but to make it easier to handle, because that amount of pure synthetic vanillin is so small, you'd have trouble getting it all out of a sachet if it were there by itself.
You can recognize that this is the case if your recipe calls for what seems like a normal amount of sugar, and separately says "1 vanilla sugar". If this is it, you don't need to add sugar at all. Just take vanilla extract (or whatever your preferred form of vanilla flavoring) and add an amount which seems appropriate to you for the batch size you are making. Your main flavors are apple and strawberry, so if in doubt, stay on the low side and add a bit less vanilla.
The order of mixing can change as well. Vanilla sugar is added with the dry ingredients, but if you are using a liquid form of vanilla, it goes into the wet ingredients. If you want a vanilla bean, the best thing to do is to boil the seeds in one of the liquids (usually milk, if your recipe has it), then cool it down to room temperature. If you add them unboiled, they don't really release the flavor.
"Vanilla sugar" can also have another meaning. Sometimes people leave vanilla pods in a container of sugar for months and then use this subtly flavored sugar as the main sugar in the baked good. If this is the case, then the recipe needs to specify exactly how much sugar is needed, there is no good way to guess. As this kind of vanilla sugar does not come in packages but is made at home in whatever amount you want, "1 vanilla sugar" makes no sense in this context, and you should assume that whoever wrote down the recipe forgot to add the unit.
You can recognize this if you either have no other sugar in the recipe, or have a sugar alternative (such as corn syrup) in what looks like a low amount for the recipe. If this happened, the best thing is to throw out this recipe and choose one of the hundreds of other crumble recipes floating around the Internet. Re-engineering a faulty recipe to a decent state is hard to do and requires trial and error, so it's normally not worth the effort.
If you are sure you need this type of vanilla sugar and are very determined to try this recipe, the best way to go is to try it out with the most common unit in the recipe's culture and see if it works. In a US recipe, this would be one cup of sugar plus as much vanilla as you think is suitable for the recipe. Here you'll have to rely on baking experience, and depending on the recipe source and having worked with similar recipes before, see which unit is likely to have been meant.
The recipe is clearly deficient if it does not provide a unit for "1 vanilla sugar" - teaspoon, gram, cup, pound, kilo... Extract plus sugar will not be much like actual vanilla sugar, IME.
I might guess a cup and a tablespoon of extract, but that's all it would be, a guess. A better guess might or might not come with the rest of the recipe to look at (i.e., is it in cups, or grams/kilos...? How much butter/shortening is there? Is this the only sugar?)
It's hard to put in too little vanilla, it is much easier to put in too much.
Many fruity recipes call for vanilla and it enhances the fruitiness in much the same way salt enhances savory dishes. Obviously it's not the sugar being added that counts in this recipe. A packet of sugar in a recipe going into a 9x13 would have no effect at all.
The vanilla sugar packet has very little vanilla in it, so I would probably go with something as little as. 1/8th or 1/4 teaspoon of pure vanilla extract and try the recipe out on a practice run. It will probably be just right, but if it's too little it will still probably be good. If you out too much in, it might become too potent to enjoy.