If you hold jump next to a vine (not at feet, but at head level), it will enter fly mode, however when not next to a vine at all, you will have to double jump.

To reproduce: build 3 blocks up, on the bottom block, place another block.
Put a vine on the topmost block. Stand next to the vine and HOLD jump.

I have not tested this on preview/beta 1.19.0.32/33 yet, I will update later when I have the time to test it (if the bug still exists)