Okay, here we go...
Looking up vBulletin spoiler tags, I found something that just may work. Here's the basic thingy:
Help me test it!
This version of it is short and sweet. I'm not going to modify it aesthetically until I'm sure it works how you people want.
The tags are [spoiler=id]Spoiler content[/spoiler]. That "id" may have to be unique on EACH page in the forum, not just in each post. Please play around with it, don't yell about how it looks, simply see if it functions like the majority of you would like it to work.