I have googled for Vimto ingredients and found them on a supermarket site.
There is one secret ingredient which the producers guard as a trade secret,
and has been used since Vimto was first made.
Since the formula originated before the days of food additives, I expect it to be one of the 'herbs' in the list below.
Water, Glucose Fructose Syrup,
Fruit Juice from Concentrate (15%) (grape 12.0%, Blackcurrant 1.7%, Raspberry 1.3%),
Sugar, Citric Acid,
Vimto Flavour (natural Extracts Of Fruits, Herbs, Barley Malt & Spices),
Colour (anthocyanins),
Acidity Regulator (sodium Citrate),
Preservative (sodium Benzoate, Potassium Sorbate),
Vitamin C,
Sweetener (sodium Saccharin).
The villain in this list is
sodium Benzoate - a preservative
which has been the subject of safety concerns because it damages DNA in living cells
see-
Caution: Some soft drinks may seriously harm your health - Health News, Health & Wellbeing - The Independent