In order to add width equals to its superview height we can do following
NSLayoutConstraint(item: view, attribute: .width, relatedBy: .equal, toItem: view.superview, attribute: .height, multiplier: 1, constant: 0)
or we can do it from interface builder
But how can I do the same using VFL?
I don't think you can, since that's a pretty uncommon setup. Esoteric use cases like this are where it's best to use the explicit initializer on NSLayoutConstraint.