What happens if I use vegetable oil in a cake?

What happens if I use vegetable oil in a cake?

I want to bake a cake out of baking mix bought at a shop. The mix already
contains flour, baking soda, vanillin, other minor components. The manual
says I need to add butter.
I'd like to use vegetable oil instead of butter - most likely sunflower
seed oil. Is that a good idea? Should I expect any problems? Will the
result likely be a decent cake or something to just throw away?