This is all new to me. How does this help with the google custom search?
BTW - I have a custom search page set up and ready to go as soon as this is implemented.
bzcomputers would be better suited to explain that, since I've never replaced the vB built-in search engine with the Google Custom Search engine. All I know is that the ability not to edit templates to make GCS work with the SiteLinks Search feature is something that was requested, and like a good code monkey I make it happen
...and it is appreciated
Let me see if I can explain it all, as far as I understand it - and see it working.
This is what the latest Google Search will look like if you have been updated by Google to show Sitelinks:

Being updated to show Sitelinks even after having the correct markup on your site can take some time, weeks to months, depending on how busy and large your site is. The SiteLinks Search option is new and just recently added to Google's Search Engine results, it is also now coded into DBSEO and you can benefit from it when your site is approved for Sitelinks also.
Why replace the built in vB Search with Google Custom Search?
To gain the best benefit of from Google's Search Engine in general and it's implementation of Google's Sitelinks and Sitelinks Search on a vBulletin site that is not stock (you have mods / custom pages that did not come with the original vB installation) you should implement Google Custom Search to replace the default vB Search on your site. What this allows for is to show search results for all the non-stock vB mods /custom pages that Google has indexed, right on your own site's search (with images and Google's familiar formatting). If you do not use Google's Custom Search and use vB's built in search you will not get any results of mods / custom pages to show in your on-site search. For some of us that is a substantial number of pages - hundreds to thousands.
With the new DBSEO coding for SiteLinks Search implemented this now allows for a direct link to your "on-site search box" to show right after your main search result and before your SiteLinks Search results at Google.com (see image above). This search box will allow users on Google.com to further search your specific site right from Google. The search box is linked to whatever on-site search you use, default vB or Google Custom Search. When a user types into that box they are redirected directly to your site and given the search results on your site just as if they had searched from your site in the first place. Again if you have mod / custom pages on your site and
do not use Google's Custom Search those mod / custom pages will not show in your results. This is another way for Google to get users to implement their Custom Search on their sites and it's also a great way for those of us with many mods / custom pages to get better indexing and better overall search results.
Hope that made sense. If not let me know.