I am encountering a situtaion where I download a Vassal module from the inventory here (in this case, Russian Campaign TRC_4thEdition_v1.1.4 the most recent one) but Vassal will not open it. After downloading it simply will not open. I get an error message saying the module is not a valid vassal module. It has to be a valid module, since its in the inventory as the most recent and widely played module.
The module downloads and opens fine for me; as expected given what you say.
Looks like something is going awry in the process of your downloading the file and opening it in Vassal. Can you share a link to the exact file that you are trying to open (via dropbox or whatever).
I’d bet a hefty amount you were using an older version and attempting to do File → Import Module (instead of File → Open Module, which is correct). This threw some users for loops literally for years, because it’s for importing from the Aide de Camp 2 format. That choice has been renamed and moved to the Tools menu now.