Importing an old cp1251-encoded forum I've noticed that the importer tries to guess the forum's encoding for charset conversion even if "Force Character Set" option is specified. The problem is, the guess was wrong in my case and I had to patch the importer's code to get it right.
I suggest to change the code so that charset will only be guessed if it isn't explicitly specified by the user.
This is really as designed - the character set is resolved based on what the settings in vB are using. The "force character set" option is only if you're forcing a connection type in vB. If the conversion is wrong, changing the character set for your vB language should be changed, not the "force" option in the importer.