1. does not delete any data means if you install into the same database and table prefix all you are doing is updating settings which needs to be updated you can not insert data for rows that already exist in mysql
2. This depends on your. Remember your switching your forums before you begin the Photopost part and doing your import from one forum to another. When you reinstall Photopost your keeping the photopost database the same as your installing into the same tables. Now as to whatever database you use for the forum that depends on how you install
vb in its own database or the same. You just enter the proper details under user database on the install config portion to connect to vbulletin database when reinstalling.
If you need us to reinstall photopost after you do your forum switch if your not comfortable doing the gallery reinstall just simply purchase an install service.