Tomato Soup substitute in cake

Agar...derived from seaweed. Used in McDonald's milk shakes because it's vegetarian and cheap, lab petri dishes and vegetarian cooking. Doesn't have a flavour. You can find it in health food stores but I think many grocery stores are carrying it these days. Heat it up in water like gelatin and when it cools you have jelly.

You could use Arrowroot to give a slight thickening for texture or even cornstarch but I'm thinking those won't give you quite what you're looking for though Arrowroot will be closer.

I think there is a Chia seed gelatin too but don't have any first hand experience with it. Chia seeds when soaked in water give off this thick gelatenous substance, very weird to see it happen. Ask about it at the Health food store too.

Here's a website for a few more substitution ideas.

http://www.vegkitchen.com/tips/vegan-substitutions/

Good luck!

To replace the tomato juice your primary concerns will be the flavor component the tomato adds, as well as the textural element. Any numbers of vegetables, steamed then pureed will give you the texture; I would recommend cauliflower for creamy, butternut squash for smooth, or carrot for spicy dishes. In terms of subbing out tomato in gazpacho and trying to retain its character this is not quite possible as it is central to the dish. However going with beet would give you an intense soup, a cold potato cream would also work well, as would any other vegetable; however they would not really yield gazpacho