Has it been made clear whether client states can be used by European nations for, say, the states in India? You can't call into wars the protectorates there and you can't vassalize them either unless they belong to an advanced tech group, which Indian tech group aren't (which most states there belongs to). I hope there is a way to make states in India subject to your European nation but still be able to call into war.
EDIT: Alternatively, create a special type of colonial vassal state for that purpose, perhaps make it only usable on Indian states. Call it 'satellite', 'princely state', or something like that.