I just did a fresh XF installed then imported my vB 4.2.x database over. I chose "Retain imported content IDs" when I did the import. Everything is good and working fine. Now I want to import my PhotoPost Pro gallery over to XFMG.
My question is do I choose "Retain imported content IDs" again for this import?
So there is no wrong or right way to import PP Pro? I'm still not 100% clear on what "Retain imported content IDs" is doing for a fresh vB to XF import. Is it so my indexed search engine pages will find the moved/imported content if I use the 301redirect file and point it to the archive table?
If I do not use that method, I noticed there are vB URL redirect add-ons in the RM. Personnel preference or does it have an advantage over the other?
It's personal preference and it depends on what your plan is for redirecting old URLs to new.
If you retain imported content IDs then a thread with ID 100 in vBulletin will have the same ID in XenForo. If you don't do that, then the thread with ID 100 in vBulletin could be imported as thread with ID 90 in XenForo.
For the forum, if you're using something like the redirection scripts, then it doesn't matter too much because they handle the mapping from an old ID to a new ID.
If you didn't have a standard URL structure in vBulletin, e.g. because of using something like vBSEO then it might be that you have to write your own custom redirect rules which might be easier if you retain the content IDs.
It's a similar thing for the Gallery. A picture in PhotoPost with ID 100 could be imported as something different into XenForo Media Gallery.
There are currently no redirection scripts available for redirecting PhotoPost Pro URLs, so retaining the content IDs may be more important here as it would make writing custom redirects easier.
You are likely going to still need that resource to help with redirecting the forum URLs regardless otherwise you would have to write all of the redirects yourself. That is a lot easier to do with the "Retain imported content IDs" option, but as you have a pretty standard setup, the easiest thing to do is just use the URL redirection add-on in the RM.
Because there's currently no similar add-on for redirecting the PhotoPost URLs, though, I would recommend still using the retain content IDs option for importing the Gallery.
So in summary:
Import the forum with or without the retain IDs option.
Import the gallery with the retain IDs option.
Use the redirection add-on you posted about for redirecting the forum URLs.
If redirecting the Gallery URLs is important, you will have to have those redirect rules written manually.