To be fair, i don't thin it's about vassal states or not. Its all about interests. I don't think you can call countries who signed RCEP free trade deal with China like Australia, Brunei, Cambodia, China, Indonesia, Japan, Laos, Malaysia, Myanmar New Zealand, the Philippines, Singapore, South Korea, Thailand, and Vietnam all Chinese Vassals. You have to know that Countries will always for what they think will serve their interests and their people grow irrespective of what others might think(even their allies). Note that the U.S pressured Australia , S.Korea and Japan not to join RCEP, however those countries knew it was in their national interests to do so, reason they brush aside US objections and join the grouping.
Same with SCO countries like Kyrgyzstan, Russia, Tajikistan,, Uzbekistan etc. They have close friendly ties with China but nothing like Vassals. As i have said before, as long as countries have shared/common interests then they will come to some kind of understand and work together on that particular field. So saying they are vassal is wrong in my opinion. National interests trumps everything.