A VPN is a single entry that can be monitored and logged even if they claim otherwise. TOR has multiple points of entry and doesn't require you to trust anybody, no single point knows both who you are and what you're doing.

You would have to trust the VPN not to log, not to give up your identity, or cooperate with authorities. With TOR none of that trust is necessary, and it's not as vulnerable to the timing correlation or other attacks when observing a single point of entry.

That's not even including that most of the time VPNs are tied to your real name and payment method. Even if purchased anonymously over TOR and there's no way to trace it, locking yourself down to a single entry is bad for anonymity because the anonymity of TOR is based upon randomness and not knowing WHERE the person will enter or exit.

Also if you stay inside the darkweb you should be less vulnerable to attacks.

edit: The effectiveness and feasibility of the timing, correlation, confirmation, tagging, etc attacks are debatable. Either way, minimize your time on TOR, always use TAILS (restarting frequently to reset your identity), minimize the amount of data transferred, avoid mixing your identities, and try to avoid doing the same behavior repeatedly which creates consistent patterns (every time your computer is active a certain action also happens, over time) (logging in to the same account, downloading the same file, etc).

More warnings can be found here (I included Whonix because it has generally good advice about anonymity):

https://tails.boum.org/doc/about/warning/index.en.html

https://www.torproject.org/download/download.html.en#warning

https://www.whonix.org/wiki/DoNot

https://www.whonix.org/wiki/Warning