The current build of Vivaldi (based on Chrome 96 https://chromestatus.com/feature/6279906713403392) should include bfcache, but at least the demo outline here:
https://www.youtube.com/watch?v=eusKoHPAWtU
...navigating example.com <-> cnn.com <-> theverge.com is not demonstrating that bfcache is enabled. Does anyone have an idea why this is not enabled yet? Using developer tools > application > bfcache and status is always shown as unknown...