red velvet cake - cake flour?
Cake flour is milled more finely than all purpose flour.  It also has a lower protein content, so less gluten is produced in the final product.  It is ideal for achieving the soft crumb of a cake.  That said, plenty of cakes are made with all purpose flour.  You would be able to perceive a difference in a side-by-side comparison.  In your own kitchen, with one cake, it is not going to matter too much.  I would suggest you sift your dry ingredients and move ahead with AP flour.  It will not be a problem.