The Vita sold around 70% of PS4 in Japan, which is insane compared to NA and Europe at ~8% and ~13% respectively. (VGC numbers) Both are near the end of their life so I use current figures.
To put this more into perspective:
After 9 quarters, the Switch has sold around 71% of 3DS in Japan, NA and Others at 136% and 134%. (Nintendo shipments)
So we can definitely say that the Vita massively overperformed in Japan due to being portable. The Switch on the other hand is underperforming in Japan, most likely due to price. It is not guaranteed that Nintendo will do more than 1 price cut, if that.
None of that discounts that portability clearly isn't the only factor as Vita still failed to outsell PS4 in Japan.
Portability is clearly the only factor that Vita had as much sales as it did. Put it this way if the PS Vita was a home console it would have sold as much as PSTV.