Can you share your encrypted vim file with others? (Honest question). Keepass can also enable browser autocompletion. Kbdx is actually very much underspecified, but it's not terribly difficult to reverse engineer apparently. The main problem (iirc) is that the most recent format version has extensions that very few clients understand.

It's popular because it's the least common denominator for "cross-platform portable encrypted key-value local storage". The sync support missing is actually a feature for most users. There are much better alternatives when you trust a third-party server.