You won''t have to worry about data loss and seo. But I understand your concerns and worries. based on personal experience and from what I have seen in 99% of cases members liked or better said, welcomed xenforo. It brings a lot of nice features for the end users which once they get a feel for them they will want more.
Getting to know xenforo is very easy from both ends, as an admin and as an user. There are a lot of tutorials here to help you get started. And if you would need to know anything you can also ask in the forum here. There are quite a lot of very helpful users here, besides the staff, who love to help out.
Did you try the xenforo demo? The backend if pretty straight forward. The hardest thing I found on the front end is for me to stop clicking on things. Half the time all you have to do is scroll over them.
As imports go, the import from vB to XF is incredibly thorough. It even covers the little things like permissions, user prefs, inline attachments, friends lists, etc. Most importers aren't that thorough.