I have been experiencing something similar although I would not call it vandalism, just changing things that are official statistics to something that is not and merging items that are not the same thing. I have recently been importing information on the UNESCO Biosphere Reserves to Wikidata with Navino Evans that includes referencing everything to the official site descriptions on the UNESCO website (he's been doing all the actual work, I mainly offer manual labour and make the tea). People have been replacing the official statistics e.g the coordinates submitted to UNESCO as part of the inscription process with coordinates with no references and also merging sites that are not the same thing (usually similar sounding sites in China and Russia). This is annoying for two reasons:
1. Its making Wikidata less accrurate
2. Its breaking our import spreadsheets meaning we have to use queries to guess at possible reasons why things have broken which takes a really long time and then I have to fix them.