Minor Faction Vassals and Puppet States

The current system lets you gain a wide array of bonuses for having a minor faction's opinion of you be sufficiently high, however even if you have had them at 100 for, say, fifty turns, someone else can just swoop in and assimilate the bastards.

My suggestion is that when one could assimilate them, it should also be possible to:

1. Make them a permanent vassal, or at least until someone else manages to sway them. In this case, you should receive a warning, so that you may implement countermeasures.
2. Invade them, but instead of assimilating them you implement a puppet state loyal to yourself. As with the previous option, it should be possible to compromise your control, perhaps my instigating a rebellion, but you should receive a warning.

The purpose of this would be to allow for a playstyle that retains control over minor factions, but doesn't force you to assimilate them; perhaps because you believe in the racial/cultural purity of your empire, you already have too many systems under your direct control, you are playing the Vodyani but don't want to kill everyone, or their bonuses/ideology is just incompatible with your playstyle. (e.g., why would a pacifist empire want to integrate a bunch of violent lizard people into their society.)