I make a vanilla cake that is a medium crumb with a light and fluffy texture. The problem I am having is that after it sits over night or is refrigerated it becomes very dense. Is there any way to prevent this? Thanks!

Are you using a box cake recipe? I'm not sure what to do if you use a box cake, however, for my scratch cakes I add to my recipe about 2-tbsp of oil. The reason is, butter in the cake when return to room temperature remains solid (I'm not sure if I'm explaining the science right), but a little oil in the batter helps to return the cake to almost the same structure it had right out of the oven. This I found after a number of trials and lots of errors. In this state you can even pop cake in microwave for about 5-seconds (i.e. without icing) to restore the texture, the results you will notice if you have a little oil in the recipe.

Hope this answers your question.

Cheers!