Yes, VoIP via UDP works very well through Tor with OnionCat. However, both endpoints must be running Tor and OnionCat. Basically, all devices running Tor and OnionCat have IPv6 addresses in the same /48 subnet. So any such device can connect with any other. But _not_ with any other IPv6 (or IPv4) address.
It's entirely possible to have some comms peers on clearnet in same chatscreen panel mashup at the same time with peers on onioncat/48, i2p.etc. It all depends on the routing horizons, and or hub selections, users setup and maintain in such conferences.
Yes, you can route VPNs through Tor. However, I don't recommend doing that except in Whonix.
Whonix is quite reasonable interesting isolation idea over Tails. Be sure to evaluate the two in own usage models.
You could use a commercial VPN service, or run your own VPN server in a VPS. But either way, you'd need to do everything ~anonymously through Tor. Or in other words, your anonymity using the VPN via Tor will be limited by how anonymously you paid for the VPN service or VPS. So that means well-mixed Bitcoin, or better perhaps Monero.
Imagine an overlay network that offered VPN termination services to clearnet from the overlay... even cryptocurrency supported ones, such as cryptocurrency data storage networks do today... the models exist, explore them. Note also, Zcash style zero-knowledge is still quite equally valid to this day, among other competing privacy crypto models such as Monero. Explore the entirety of onchain privacy, it is evolving nicely, early days.
All things considered, it'd be simpler, and arguably more anonymous, if you can force Jitsi to use TCP via plain Tor.
Not against State Associated Agents, such as say LE conjoined with Top Secret entities Parallel Constructing, or other generic Sybil trolls, perhaps, as in the subject Snowden and other slides and public docs, this hardly in fact true, tor being no exception, witness myriad published tor network exploits, same as any other current overlay network. Against random wannabe's, such as say an employer, or some website, sure, reasonably well. Those two, and others, are still a good thing, accomplished by any fairly rudimentary overlay measures. That covers many users, but not all those rightfully, justly, and naturally, freedom and privacy oriented. Also, Tor Project Inc snowflakes are censoring this post by forcing it to go to mandatory "moderation" first, while others are free to post entirely unmoderated, including, shamefully, said self appointed TPIs. Think of the ramifications before you accept such censorship regimes in your life. Move to distributed messaging, social, and multimedia to ensure your freedom of speech and consumption against such censors like TPO, Youtube, Reddit, Facebook, Instagram, etc... ASAP!