I had this problem when I went from vB 3.8.4 to IPB last year - it didn't seem to matter what I did, it simply took time for Google to reflect all the changes. It levelled out after about 4-6 months.
I take it your site is running nice and fast and Google is able to crawl your server at a good rate?
Mostly this is due to the vBulletin v3 "archive" (an SE friendly presentation of all forum content, text-only). Most SE target content from there and this is lost completely after the migration (because redirects don't handle the "archive" URLs).
In fact, I had to make doubly certain we redirected our archive URLs since I disallowed most robots from our main site. When we were having an overload of traffic on our older server, the bots hitting the full site were really slamming it hard. Shifting all that bot traffic to the archive (text only) view helped us a bit.
I think that google is recalculating the rankings and this will take time. I hope to come back with my rankings in the next weeks. ;-)
That doesn't mean it's XenForo's fault. Did you redirect old links? you need to keep in mind that Google has also made changes to its algorithmAfter two years using the XF can say that for my website, the replacement system was the worst thing I did before our traffic was 25,000 visits per day, now barely passes 7,000 visits per day.
What I realize is that XF near another simpler systems is much higher, as seen near the IPB and VB the comparison is not up to snuff ..
Again, this is from my point of view, by my experience, if it worked for others I'm happy, but for me the result was very bad.
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