Is there any note taking, or password vault app which can work offline, cross platform, sync data with online storage services, like Dropbox, Google drive etc., and do encryption?

I use KeePass for some time now and it works well. It runs on many platforms and for sync you can simply put the password file into the Dropbox folder.

@Letmecode That's why you encrypt the passwords.
Use a large number of key transformation rounds and brute force will take ages.

I use KeePass and Syncthing.

Mac: MacPass & Syncthing
Windows: Keepass & synctrayzor
Android: Keepass2android & syncthing

Or try Enpass https://www.enpass.io/

Grt thanks. KeePass is really cool. The reason i asked bcz i was creating an app to save encrypted notes. So i was confirming if I am not wasting my time to make such app.

KeePass2 works very well with drive/OneCloud/OwnCloud(if you are paranoid)/USB stick(if you are ultraparanoid)

There is a vulnerability, but it's accessible only when the client is open and the file is decrypted, in other words someone has already access to your system and your screwed anyway.

Used KeePass with OwnCloud/Drive and it has performed 100%

I also found web based client, which is fast, looks nice. And i believe works offline... i am still testing it

Keeweb.info

And https://keepassxc.org