What that means changes on the threat model, but for this scenario we'll say a hypothetical VPN provider in a different jurisdiction that takes Monero and has good OpSec.
The reason they can do the deanonymization attack on the Tor circuit is that they have basically unlimited resources to set up nodes, and Tor is built on the assumption that the nodes are all unrelated.
If a Tor session looks like:
User -> VPN -> Tor( A -> B -> C ) -> Market
then the government could have the advantage of owning both A and B, or any other combination. That is difficult (though not impossible) to do to a VPN that they do not have jurisdiction over.Anyone can set up a Tor node, but it's not possible to just randomly add nodes to a VPN's network, so they shouldn't be vulnerable to that (barring insecurity in other forms).