Depends on the exact flavour.
There are two vegetable varieties:
Slim a Soup, Cajun Spicy Vegetable, dried, 16.5g sachet = 2.5 Syns on all plans
Slim a Soup, Golden Vegetable, dried, 13g sachet = 3 Syns on all plans.
All of these are Chicken flavoured:
Slim a Soup, Chicken & Leek, dried, 11g sachet = 2.5 Syns on all plans
Slim a Soup, Chicken & Mushroom, dried, 14.5g sachet = 3 Syns on all plans
Slim a Soup, Chicken & Sweetcorn, dried, 14g sachet = 3 Syns on all plans
Slim a Soup, Chicken Noodle & Vegetable, dried, 15g sachet = 3 Syns on all plans