Installing plugins, etc, with numerous styles already installed
 

If I've got a plugin or other modification, would I have to apply it in every style I have uploaded, or does vBulletin apply the mod regardless of what style a user chooses?

I've only been using vb for a few days, but I'm presuming I'd need to manually re-code all styles where the moidification updates source code, but wouldn't need to if it's just a plugin for the AdminCP panel...?

It depends. Child styles will inherit modifications made to parent styles as long as the template is not customized in the child style.

For example if your styles are set up like this:

- Master style
|
|- Child 1
|
|- Child 2

Any modification made to a template in the Master style would work in the Child styles. However if you've set it up like this:

- Master Style
|
- Child style 1
|
- Child style 2

You'd have to edit the template for each style.

i have that setup too.. but in my case.. not all child templates work when taken of masters.. so i had to edit them.. and every time i install plugin and it doesn't work u have to tweak the templates in child that are different..