Should I top up pickling cabbage solution? I'm making a first attempt at pickling cabbage. I compressed some cabbage in a suitable container, and added hot vinegar with spices to it, making sure to fill it right up to the brim. I put it in the fridge.

The next day I take a look at it, and presumably the cabbage absorbed some of the liquid, since there's a 5cm or so gap at the top.

Should I add more vinegar to it and top it up? Or just leave it as it is?
Generally yes you should top up the solution, anything exposed to air even in the fridge will start to spoil.  Many pickling jars have an insert and/or weight that you can add at the top to make sure the food remains submerged.