Close empire relations lost due to becoming a Vassal

[arvcran2]

I had an unanticipated event, which, has caused the loss of a close relations with another empire of the same race.

I was considering what it would take to declare on the empire that demanded my ally become their vassal, to capture a colony that was taken from my ally by them in order to trade it back to my ally for them have a chance at growing and being a nuisance to the aggressor.

All the previous treaties were cancelled. Now instead if I declare on the aggressor, it will cause my previous ally to declare on me and, as a result create opportunity to amalgamate their capitol into my empire.

See AAR for more context

Am wondering if it is possible to free the Vassal from the story wise 'oppressor'? And if so, how to go about it.