Unless you’re touring Norway all expenses paid (with a tour operator, for eg.) or you belong to a royal family somewhere, be prepared to shell out big bucks for eating out in this gorgeous country. Vegetarian or not. We traveled with a big crowd and ended up cooking most of our meals in our apartment hotels, but even with that condition, eating is a big deal here. Groceries cost twice or thrice what they do in the U.K. at the minimum. (Staples such as milk and eggs are at least twice the cost while indulgences like fruit and veggies cost much more.)
That said, eating vegetarian while on the go is about as easy or hard as finding the next pizza place or Indian joint. In bigger cities like Oslo and Bergen, most people speak English well, so ordering, say pizza toppings, isn’t tedious at all. In smaller towns, locals may not be as fluent but they more than make up for it in friendliness. A bit of pointing and nodding will likely get you the food that you intend to order. Bonus: Norwegian beer is nothing to scoff at! If nothing else works, at least you’ll be sloshed enough to not notice.