Yes it penalizes for that, when googlebot or anything google related that actually runs the test visits a site, it visits the primary domain however you can input either url the primary or the /forum to check and it will gladly verify if its mobile friendly or not. I'm dealing with a site now that uses vBa CMPS and its not mobile friendly, there's actually a few work-around's but nothing solid yet, here are some hopefully helpful links:
https://vborg.vbsupport.ru/showthread.php?t=285520
http://www.vbadvanced.com/forum/show...e-Mobile-Style
This is the same info as link #1 just on their site however a good discussion takes place:
http://www.vbadvanced.com/forum/show...e-Mobile-Style
Needless to say the site I'm working on is awaiting an upgrade by their host, then I'll be able to test, if I find something helpful I'll come back and share

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