You seem to have forgotten what the topic even is. Metacritic is totally irrelevant. We're talking about what sells in Japan. Look at Japan's software charts, look which system's games take nearly every spot on the chart; the Switch.
Allow me to answer the question for you; Switch is selling better than the Vita primarily because it has a far better library. That's what makes the difference between a portable selling less than the PS4 (Vita) and more than the PS4. (Switch) The games.
I didnt answer your question because i thought it was obvious when i said vita had no 1st party and 3rd party AAA games. You need to pay more attention to previous posts. Also the wii u has many of switchs 1st party games so why is switch on track to triple quadruple the wii u in sales in japan?