Depends on what you mean by stacking profiles. Devices will add up, but there is no automatic merging of cloud-init.user-data sections. This means only cloud-init.user-data section from the last profile will be applied to an instance.
But cloud-init.vendor-data uses the same format as cloud-init.user-data. So, as long as you don’t use the same config keys in both, cloud-init will apply them together to the instance. To sum up, only keys from the last cloud-init.vendor-data and cloud-init.user-data sections will be used.
Unless you set up a proper merging of cloud-init.user-data by hand.