I've lived in vans for well over a decade full-time and travelled in them year round since 1975.
In all that time the ONLY time condensation was ever a serious issue was camping in an uninsulated van in 2 feet of snow. Then we had condensation. It caused no damage whatsoever because it was only short term. It won't cause damage unless you have broken paint on the inner walls of the van and you cause the condensation repeatedly.
The rest of the time, the worry that someone who does not live full-time on the road has is vastly overdone. If you plan on living in the van full-time in a cold, snowy environment like spending the winter in South Dakota then go ahead and plan for condensation reduction and mitigation...otherwise it is a non-issue.
Better mitigation like ventilation several times a day and prevention of large amounts of condensation will go a whole lot further than trying to install a vapor barrier. There is a reason, and it's not lack of expertise and cheap construction that NONE of the RV produced commercially have a vapor barrier installed.
You are in and out of the van multiple times a day, the air transfer of opening and closing van doors equalizes the temperature and therefore the humidity without any further action.
Use of a vapor barrier, IMO, is uncalled for and in a lot of cases, causes more harm than good. When I say that it is because a good vapor barrier (100% sealed) will prevent natural humidity caused by cold air outside meeting any kind of warmer air from dissipating. It will collect on the cold side of the vapor barrier - that would be between the van wall and the vapor barrier where is will cause problems. An incomplete vapor barrier won't do any good so it's just a waste of time, effort and money. Doing a 100% vapor barrier in a s&b home is difficult enough and has proven to be problematic in other ways, achieving one in a van would be almost impossible.
Unfortunately a lot of people with no actual on the road experience and only text book or youtube/google knowledge try to envision what they think are possible scenarios and then offer advice based on what they think instead of what they have actually experienced.