The security aspect has already been mentioned. That is also my main motivation. Another reason for me: minimalism - installing as few apps as necessary.
I used to use Brave and Tor and then just tried Vanadium (I had read a lot of good things about the security and the browser was already pre-installed on GOS anyway).
Currently I use Vanadium exclusively (in a kind of test phase) and, if my purpose requires more anonymity than security, the Tor browser.
I miss some Brave features (such as the synchronization of bookmarks and the blocking of annoying ad trackers, integrated wayback machine), regarding other features (Brave rewards, wallet, VPN and other bells and whistles (not meant pejoratively)) I praise the simplicity and clarity of Vanadium.
I still see some significant advantages of Brave over Vanadium in terms of resistance to fingerpriting - I've already mentioned this here.
But all in all, I am currently very happy with Vanadium and see no reason to replace it with Brave (or any other browser).