Even if someone got your vault file, with a _very strong_ master password it's just not going to get brute forced any time soon. [1]
With an online-only solution you have no idea how they are storing your data. I think 1p local vault (only) with db sync with an extremely strong master pw is adequate, but indeed for most use cases, it could be better to simply one-way sync from your main computer to your mobile device with something like Resilio Sync and avoid Dropbox entirely.
I cannot bring myself to trust any online service with this kind of data. Nobody is getting my master password without hacking my machine, brain, or government backdoor. There is a lot of peace of mind to be had with a local system IMO.