For whatever reason, it's not always possible to find an old thread using the vBulletin search form that easily. To combat this, you can subscribe to any thread you want to reference in the future. However, subscriptions can be used for your own threads too so there's no need to start a duplicate because you can't find your first thread for example.

At the top of the thread, you will find a Thread Tools menu as highlighted in the screenshot. All you need to do is click on that and then click Subscribe. It really is that simple:Click image for larger version. 

Name:	Screenshot_20171216_192452.jpg 
Views:	50 
Size:	74.0 KB 
ID:	29263

To find your subscribed threads, open your user control panel. That's the cog icon at the top right of the forum next to the logout one, then click subscriptions on the left. you can organise them in folders and delete any that you no longer need from this view:Click image for larger version. 

Name:	Screenshot_20171216_193156.jpg 
Views:	38 
Size:	80.5 KB 
ID:	29264