Take F225 - much better views and no problem for Niva.
Not clear what you mean "How difficult is the drive to Landmannalaugar from the west, then continue on to Vik?"
Vik is to the south-west from Landmannalaugar "across" Myrdalsjokull.
But the drive shouldn't be a problem...well depending on your experience of course. I mean whether you're used to highways only or drive unpaved country roads all the time.
You can see by yourself what you can see overthere. The story starts from Skogafoss and ends at Landmannalaugar. Then it goes across highlands, passes Vik etc.
Photos from June 26 - July 5 this year.