“Fatal error: Cannot redeclare my_add_null_slashes()” means the script that contains the definition for my_add_null_slashes() has been included by a parent script.

I hope the "vitals" you mentioned is "include/lib/vital_funcs.inc.php" instead of "include/vitals.inc.php". The latter contains session operation and some outputs that you don't need.

In terms of where to include "vital_funcs", it seems more reasonable to include it into the particular installation (or upgrade) step, for example, step5.php or ustep4.php, since at this point you are dealing with a particular step. "install.php" is the wrapper for all steps so the inclusions there need to be fairly generic.