Threadmarks is an add-on for vBulletin and the owners have not made it available. The add-on is at the forum level so you can't do it on your own.
Firefox does "tagging" which works good and is quite elegant. I use Chrome mostly. I do the following:
1) Create a new Bookmark folder
2) "Star" or "CTRL + D" a thread that I want to have tagged in Chrome
3) Append "#whatever" to the title of the site as I'm naming the Bookmark, where "whatever" is the tag name I like. You can append multiple times
4) Save the bookmark in your created folder or wherever you'd like
Now, you can either search the folder or just type the tag name into the address bar and your saved bookmarks with that tag or tags will show up.
This actually works well, especially if you are focused on some key areas and you want to review curated, bookmarked threads with tags that you won't likely forget. Plus if you're using Chrome's sync feature the tags work across multiple PC's and mobile devices and the tags appear at the top of the results as you type in the search bar.
"My wife always wanted girls. Just not thousands and thousands of them......"