Preheat oven to 350F and line a 9x9 square pan with aluminum foil (enough so that the foil hangs over the sides so that the bars can be easily lifted out of the pan and cut when finished)
In large bowl, stir together flour, brown sugar, oats and salt until well-combined.
Using a pastry cutter cut in butter until mixture resembles coarse crumbs.
Press ⅔ of the crumble mixture into the bottom and partly up the sides of the prepared pan.
Bake on 350F for 13 minutes, remove and allow to cool while the filling is prepared (do not turn off oven).
Filling
In KitchenAid or with hand mixer beat cream cheese until creamed.
Add condensed milk, and eggs, whisk well.
Add in lemon juice and vanilla extract, whisk until mixture begins to thicken (about 2-3 minutes) and pour over the baked crumble base in 9x9 pan.
Distribute remaining crumb mixture evenly over the top of the lemon filling.
Bake on 350F for 15 minutes.
After baking, allow bars to cool to room temperature before transferring to fridge and chilling 2-3 hours (if bars are difficult to cut after 2 hours, return to fridge and allow to cool 1 hour before checking again).