Google's Webmaster Guidelines insist to "...Use a text browser such as Lynx to examine your site, because most search engine spiders see your site much as Lynx would...." (Google's Webmaster Guidelines)
Given this "limitation" of Search Engine bots, it is very important to make it easy for them to determine what each of your site's pages is about.
One way to do this is to use H1 tags to enclose your page's title, or a sentence indicating what the page is about.
However, H1 tags are usually big, with lots of spacing, and take up a whole line. The trick is to add H1 tags without affecting your carefully designed web layout.

Following are instructions on how to add H1 tags to vBulletin's Navigational Bar (where vBulletin shows the breadcrumbs and the title of the current page, like forum and thread titles) without affecting how your site looks.
This is one of the many features implemented by NuSEO.PHP when optimizing vBulletin communities. If your site is running NuSEO.PHP, do not follow these steps, these changes are made automatically without the need to edit vBulletin's templates.

Steps:
1) Edit the headinclude template and at the end, add these lines:

<style>
.nuseo_hx { display: inline; color: inherit; border: none; font-size: 1em; font-color: inherit; font-weight: inherit; padding: 0px; margin: 0px; }
</style>

2) Edit the navbar template and find this code:

$navbits[lastelement]

and replace it for this:

<h1 class=nuseo_hx>$navbits[lastelement]</h1>

Save your changes and go to a forum or thread in your site. If everything worked as expected you should not see any change. Right click and go to 'View Source', search for the forum or thread title and it should be enclosed in H1 tags for all bots to see.

Notice that whenever you upgrade vBulletin and these templates are reverted, you need to reapply these changes.