I've send a mail to the support team of vBulletin asking about the subscription procedure and here is what I got as an answer:
Hello,
To subscribe to a user on the vBulletin.com forums first find a post by the user. Click on their username below their avatar.
On the next page you will see a "Subscribe" button, press it to subscribe.
A screenshot of the button location is here:
https://tmpdmp.com/254cb5d954cf631d/13239f4f5202738e
Please let me know if you require any further assistance with this issue.
- Joe D.
vBulletin Support Staff
I hope this help your issue!