A stock cube should support the vegetables, not make every soup taste identical. Some are intensely salty or herb-heavy, so the packet's dilution guidance is a sensible starting point.
Make a small amount and taste it before adding it to a delicate sauce. If it is too strong, use a weaker mixture and adjust the finished dish. Remember that simmering reduces water and concentrates salt.
Check the ingredients for animal-derived components and anything else you need to avoid. “Vegetable” on the front is helpful, but the full label is the better source of detail. For a pan already containing soy sauce, salted tomatoes, or brined ingredients, plain water may be more useful than another layer of seasoned stock.