When I click on the Víkingr desktop icon, I get two launchers (no it's not a mouse problem; I've checked). Apparently only one of those will be the proper WSE launcher, and if I choose poorly I won't be able to join a server, getting the usual non-WSE error message.
These are the shortcut's properties:
Target: "C:\...\Steam\SteamApps\common\mountblade warband\Modules\Vikingr\WSE\WSELoader.exe" -p ..\..\..\mb_warband.exe -m "Vikingr"
Start in: "C:\...\Steam\SteamApps\common\mountblade warband\Modules\Vikingr\WSE"