If you do a dungeon with someone lower leveled you'll still get credit for completing the pledge. You can also still do the quests and earn your skill points, collect the lore books and loot chests and other assorted containers . You just won't earn xp or get loot drops from killing enemies.

Both Veteran and non Veteran dungeons scale to the level of the group leader. So as long as it's scaled within five levels of your character (roughly anybody lv46+) you will gain xp and loot.

Someone might correct me but as VR1-14 player you are still considered level 50 so you can keep doing it at level 45 if you wish (though you won't get much xp). I guess that's why VR is Veteran RANK not level