When I'm replying to a post which quotes a previous post, only the last post's text shows up, not the text the last post was quoting. My guess is that vBulletin doesn't support nested quotes.
On forums that use other forum software, the post I'm quoting would be nested in my reply showing who and what the reply is about. Here's what it would look like, in this case done by manually entering HTML 'quote' tags.
Here's how I'd do it manually (minus the spaces after the brackets):
[ QUOTE=Larry Browning;2562169 ][ QUOTE=Allan Speers;2561896 ]I dunno about...[ /QUOTE]I am sorry...[ /QUOTE]
Using The Google I found an add-on for vBulletin that does exactly this. It's on vBulletin.org, a site for developers who write mods for different versions of vBulletin. There may be others, but this is the first one that came up in a forum search.
I know that automatically quoting all quotes in a reply can result long posts because many members don't realize they can (or don't bother to) edit the posts they're quoting. I also don't know how comfortable our forum experts are dealing with add-ons. Still, it might be worth thinking about.