Doug McGuff, author of Body By Science, mentioned Jay Vincent in one of his videos, and so I did a search on the guy:
He claims not to have used steroids, but I think the chances of that are remote. I'd say he has at least a nodding acquaintance with the stuff. Regardless, it's worth having a look at his workout and to hear how he eventually adopted a HIT approach to training. Personally, I don't like the very slow cadence of his reps, which approximate 10 seconds each. This is somewhat more "brisk" than the SuperSlow speed of about 20 seconds per rep, but still too long and ~unnatural for my liking. For better or worse, I favor about 6 seconds per rep. Interestingly, in the interview video, uploaded in February 2019, he said that people should work out more than once a week (at around the 12-minute mark), which he reportedly did at the time, although using a split routine. However, in the comments section of the workout video, which predates the interview and was posted in July 2017, he made a comment 5 months ago (so, subsequent to the interview), that he only does this full body workout about once a week. So, understandably, his workout frequency apparently depends on whether he is doing a full body or split routine.
I also think that he still took steroids. I would also take a look at his full workout live because you can do editing so you won't even notice the difference. I guess he could have used YK11, it is very good at helping to increase muscle mass, my friends tried it and were very pleased if you can read more here https://sarmguide.com/sarms-guide/ I also noticed that in a video interview uploaded in February 2019, he said that people should exercise more often than once a week, which he reportedly was doing at the time, although he was using split mode. In conclusion, I can say that do yourself, go to the gym, and consult with trainers.