I mean routing traffic to Tor entry guards through VPN services. The Tor Project does indeed not recommend that. They argue that using a VPN service is risky, because it can log everything. Where access to entry guards is blocked, they recommend using bridges (of one sort or another) run by Tor volunteers.
I don't agree with that argument. Because ISPs can already do that. And for most people, their ISP is far more likely to be cooperating with their local adversaries than some random VPN service is.
And for what it's worth, one of Tor's inventors (Paul Syverson) has agreed publicly that there are reasons to access Tor through VPNs. Basically, when you don't want your ISP to know that you're using Tor. Indeed, if I were a CIA agent using Tor in Iran, I probably wouldn't want the ISP to know that I was using Tor.
But I don't trust VPN services either. So I use nested VPN chains. That's basically the same approach that Tor itself uses, routing traffic through multiple (three) relays. So no one relay (or for me, VPN service) knows both who I am, and what I'm doing online.
There's also the issue of trusting the Tor network. Some argue that it's compromised by US TLAs. So with a nested VPN chain between me and entry guards, I'm less concerned that some TLA is running them. But even if that's just paranoia, there have been bugs that deanonymized users.
For example, some years ago, CMU researchers exploited the "relay-early" bug to allow malicious entry guards and exit relays to exchange information, and so learn that they were routing the same circuit. That allowed said CMU researchers to deanonymize Tor users. The FBI learned of this, and subpoenaed the data. And lots of people went to jail over it. Mostly drug dealers and child pornographers, but whatever.
However, routing VPN services through Tor is a totally different matter. If you do that, your anonymity depends entirely on how anonymously you've obtained, paid for, and used the VPN service. If you used an email address that's linked to you, you're screwed. If there's a money trail in paying for the VPN service, you're screwed. If you ever use the VPN account without Tor, you're screwed.
And even if you manage all that anonymously, the very fact of using a VPN through Tor decreases your anonymity. That's because Tor by default switches circuits at ten minute intervals. But when a VPN is connected through a Tor circuit, that circuit is pinned. So by using a VPN through Tor, you've blocked one way it increases anonymity.