There are threads on vbulletin.com where problems like this were discussed, like this one:
https://www.vbulletin.com/forum/show...s+feed+problem
I don't know if anyone said for sure whether it's a problem with google or vbulletin, although it seems like a number of people said they have multiple feeds and only the google feed is a problem.
Anyway, one solution would be to set the forum to allow HTML, then set "Convert HTML to BB Code" to "No" in the feed manager. However, you don't want to do that unless no one else can post in that forum (except maybe admins). (I suppose even posting HTML from google's feed might be relying on google to ensure that it doesn't contain anything malicious).