The nature of a viral clip is that they are one shot, one time deals. I had a potential one that didn't get viral. Probably because I was too far away from the action, or just did not have the amount of exposure others have had with similar videos.
It's still the most viewed video on my channel, and in my eyes did pretty well due to the fact it gained over 10k views in a single month where my average view per video is something like 420. Please do note that most of my vlogs struggle to get above 200, so that average number is carried by my cars & coffee videos, the one "viral" video and my car reviews. I had some folk say: why vlog, no one watches them. And I just simply say: because I enjoy vlogging!
So, a viral video (or one with a lot of views) does help, but I don't think it makes for lasting subscribers.