There are no specifics, but my suspected answer will be: Yes. For both the official xenForo convert script, as well as xenPort by xenfans.com
It's already confirmed that you can set the pass scheme, so users can be imported and keep their passwords.
When xenForo is released, I believe it will come with an IPB and vBulletin 4 converter/importer. There is a slight chance that the IPB converter may not be released after the release of xenForo and maybe slightly delayed.
A vBulletin 3 importer has already been created, therefore should be released on the same release date as xenForo.
Yes - although you might not find compatible hacks/skins in the early stages of the software, so you might need to rethink your options and wait a few years if your board is heavily hacked and you want to keep all modifications.