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My personal approach would be to simply start over and see how things evolve.
If some users asks for his data, open a new discussion and ask how to do this. You will get an SQL statement from one of us which exports most of the data. Pass that to the user and see if he is happy with that (most probably he will)
If he wants his content to be deleted, use the inbuilt Vanilla feature. That is not complete, but the user will not be able to see that, only someone with access to your database might notice that fragments are left.
What you have to take care for in respect to other information stored in the database or on the server, you should be able to find general hints. There's no difference in this regards to WordPress. Every blog/cms/forum has the same needs concerning to the hosting side.
And again: if you come back with a request like "I need to purge/backup this and that information periodically", someone will provide you with an SQL that would extract/remove that data.
As long as your forum is only a hobby, I doubt that you will ever have to deal with those legislation burdens. And if you are doing it professionally, you should spare some money for a lawyer anyway.
"Title" and "Location" are special profile fields. Maybe that is because such fields are commonly available in other forums, I don't know...
In order to use this two fields, you do not need use the profile extender, only two config settings:
$Configuration['Garden']['Profile']['Titles'] = true; and/or
$Configuration['Garden']['Profile']['Titles'] = true;
If any of this fields has a value, it is displayed automatically. If you "add" this field with the profile extender and use the show in profile feature, it is added twice.
The size of the downloaded files for installation is totally irrelevant.
You said you want a "light" forum, but I guess you either mean one of
1. "fast" or
2. "low traffic for the browser"
If you have a simple shared hosting forum, I think there are faster forums than Vanilla (though I never have tested it)
Vanillas structure is comparably complex, which could make it some milliseconds slower, but brings numerous advantages in regards to maintainability, code readability and "extendibility" (don't know the correct word)
"Low traffic" depends completely on the theme, no matter which forum you choose.
I understood it that way, that this old forum is "some forum" and it should be converted to Vanilla. So the question would be what would be needed to convert.
Yes, I think some more clarification could be helpful
The hosted version has features that the open source forum has not. Therefore some functionality might be lost, maybe even some information from the past.
If your forum was using reactions, you most probably will loose that information. You could re-gain the feature with some application from the open source community, but the historical data will be lost if you are not having a good knowledge of SQL and it would take some time even if you are a SQL guru.
I think the hosted version has something like groups. There is nothing like that in the OS version.
And maybe even more features that I do not know of. I cannot imagine that you will be given the theme, although I maybe wrong. So maybe your forum will have complete different look.
But all in all transferring from hosted Vanilla to OS Vanilla should me way more easier to transfer from whatever Vanilla to whatever other forum script