Wilflower I am currently using Vanadium + Orbot. Is this setting completely sufficient for safe browsing?
Depends on your definition of "safe". This is certainly not very anonymous because Vanadium + Tor will give you a rather unique fingerprint. The standard advice is that if you require the anonymity of Tor, use the Tor browser. This has some security tradeoffs though, so it's a tough call that will depend a lot on your threat model.
Wilflower Is it worth adding RETHINKDNS to it?
I'm not sure this is even possible, since I believe both Orbot and Rethink will need to use your single VPN slot. I'm not very familiar with Rethink, though, so maybe you could do this somehow. Without knowing how this would be setup, I can't say what benefits, if any, it would provide.
Wilflower Do I understand correctly that in terms of anonymity it is better to use Tor browser + VPN?
Tor is generally not considered to also require a VPN, but there could be some minor benefits. There are 2 ways you could do this: Tor -> VPN or VPN -> Tor.
Tor -> VPN would hide the fact that you are using Tor from the sites you visit.
VPN -> Tor would hide the fact that you are using Tor from your network provider.