Without checking out every instance of characters that were not imported correctly, we so far found that the following was not imported:
Apostrophe, "(quotes), tree dots, € (euro), – (minus), α, ä, α, δ, anything greek, arabic, Chinese, Persian.
Basically any symbol and any character that is not basic latin.
There must be some way that the importer can insert the old text into VW4/XF in the same way that the stock xenforo importers insert the vbulletin content into xenforo. We do not encounter the same issues there. Similarly we imported 26k articles & blogs from vbulletin threads into AMS and 10k journals from vbulletin threads into UBS without this issue. I know for sure that members have been using the same characters there and have also employed MS Word to write those articles.
I dont know if it is a useful remark, but maybe the same concept can be implemented?
It looks like all or almost all wiki articles are affected by this issue.