Combining Tor with a VPN is tricky to do in a way that helps you, and easy to do in a way that hurts you. The fundamental problem is that you're adding a new "hop" in your Tor path, and you're hoping that it makes things safer. If you add that hop at the beginning (that is, you use a VPN to reach the Tor network) then it's harder for your ISP to realize that you're using Tor, but now there's a new company that gets to spy on your Tor traffic. Whereas if you add the VPN at the end of your path (that is, you use Tor to reach your VPN), then even if you somehow managed to make the VPN account anonymously, they still get to see all of your Tor destinations, which lets them build a profile of your interests across time.
VPNs are notorious for being security bottlenecks. That is, their centralized architecture generally means that they are great points for attackers to break in to (or bribe, or coerce) and then they get to know everything that all the users do. See this page for more details:
I think there are some situations where using a VPN to reach the Tor network makes sense. I'm thinking in particular of places where using Tor is uncommon and will make your ISP freak out, but using a VPN is normal and your ISP won't care. So a lot of the remaining battle is about educating the ISPs that Tor is more ubiquitous and more commonplace than they might think.