Ideally, the demijohn should be nearly full, but if you do that from the beginning, it's going to bubble over because of the fizzing and darting of the initial fermentation (although this depends on the yeast, the must and the temperature). The reason you want it as full as possible is not because of oxidation- the head space will be full of CO2- but because when you do come to top it up, you'll need less liquid.
I used to fill to half way up the shoulder and then use a small size freezer bag held in place with a rubber band fixed over the top Then, when the first fury of fermentation was over and the yeast began to settle, I'd rack the clear wine into a clean demi John and top up to the top of the shoulder/bottom of the neck and fit an airlock. I never used Campden tablets in my wine. Sulphur dioxide has a bleaching effect and can alter the colour of reds.
I've never used a kit, but aren't they supposed to make 6 bottles? That's a full demijohn so if you have a bit of topping up to do it means that the wine was already over the intended strength and you'll just be diluting it to what the kit-maker intended.
Topping up with vodka is a bit extravagant. If you use a lot of vodka you might take the whole over the alcohol tolerance of the yeast and end up with a sweeter wine than you intended.
So why don't I still make wine?
I prefer pints of beer. Pints of wine isn't a tenable long-term option. Also, I love the challenges of brewing beer. But if I had to make a choice of drinking material, it would be good traditional, west-country cider, made with proper cider apples, not supermarket juice or concentrate.
BUT
@bottleinfrontofme, you worry too much.
It's OK to make mistakes. Put your demijohn of wine on a tray or in a bucket, full it up and just see what happens. You might find everything goes well and you don't lose a drop. But, you've definitely got to do the learning curve for yourself.
Moral of the story: don't fork out for the most expensive kit you can find until you've got a grasp on what you're doing!
