[Dan]

1.) As of version 3.5 and newer most mods are made as "plugins" and these plugins are just uploaded through a web interface and it works. No editing files just a few clicks of a button and maybe a couple file uploads.
2.) No mods don't invalidate support, you'll be told to turn off the plugin system so those mods don't interfere with basic vBulletin product if you request support through offical channels at vbulletin.com
3.) Most installation instructions are straight forward and simple some of the bigger modifactions are a bit trickier sometimes, and most people do provide screenshots of their modifactions.
4.) Not sure, I haven't used phpBB in ages but I'm going to guess most mods are already here and if not they can probably easily be coded for vBulletin.
5.) As long as all the mods are plugins you won't have to reinstall them.
6.) Yes we will be to help you'll just have to buy vBulletin and you'll be able to post anywhere here and we'll try our best to help you.
7.) Sure?