The PSA regarding the Parent argument of Instance applies to all objects in the DataModel, not just parts. Here’s the post again in case you want to re-read it. The post itself actually has a decent enough explanation to cover the issues and reasons against the practice of parenting first before setting.
By nature of the above, it also applies to ViewportFrames. Once a ViewportFrame is in the DataModel, it’s no longer detached from the DataModel (as it starts off on creation) and internal Roblox systems start acting and listening on the object.
It’s always good practice for any objects going into the DataModel to have their properties (states) set before being added, regardless of what it is.