Liz dear, don't go leaving milk out overnight. Just add the acid (lemon juice or vinegar, as you say) and proceed with the recipe. Milk left out for that many hours is not safe to eat. It's spoiled, and we have no idea what microbes are in it.

Baking will kill the living microbes but may not affect toxic by-products which the microbes produced while metabolizing the milk.

Two teaspoons of lemon juice in a cup of milk will give you the 'sour milk' you need. I don't usually buy milk, so when a recipe calls for sour milk or buttermilk, I use plain yoghurt. That's how I make Irish soda bread every St. Patrick's Day.

And if I'm feeling slapdash, 2 tsp of lemon juice in a cup of water will do.