I have no idea how much vendors use XMR but unfortunately we cannot prevent the junkies and criminals using the best alt.
What is considered 'criminal behaviour' is very very relative in time and space, depends on who's in charge, societal climate etc etc... And yes, these people, whose behaviour has been outlawed will seek the best way to hide said behaviour... This is not something which anyone should seek to prevent, at least not by diminishing anonimity/privacy and thus fungibility in a currency
Criminal = outlawed.
If you live in Somalia which has no law and order there is obviously many things possible but if we look at the majority of countries, most of them has outlawed drugs and certain type of pornography for instance.
The criminal activity has partially ruined the reputation of bitcoin (many still think it is a currency of criminals) so it may not be good for the reputation in the beginning. Therefore, being accepted by the D.N.M. is kinda double edged sword: on the one hand it is nice that XMR has some real use case, on the other hand, it might create reputation to the XMR of being the currency of the bad guys.
This is the largest criminal asset of all time.
It has such a bad reputation no-one will accept it, right?
Well to be fair maybe gold is larger but the point still stands.
I think you are confusing with terms of crime and morals.
Dollar as a system can be perceived as immoral (stealing purchasing power by inflation) but it is not criminal because the legislators have approved it. It is also a social contract that the governement has a right to collect tax and inflation is one form of a tax, if I am not mistaken, even Ben Bernanke has admitted this.
In case you meant the use cases of dollars being used by the criminals, you are right in one way The majority of drug deals, hitman fees etc are paid with cold hard cash. That being said, the criminal use cases of Monero or bitcoin represents probably higher percentage than it represents in fiat. So it is relative also.