Small muscular VSDs are very common in childhood. they are small holes in the wall between the two lower pumping chambers of the heart. A 2.5 mm hole would indeed be small and likely will close over time. the good news is that they do not increase in size. if the remainder of the heart anatomy and function is normal, then you have little to worry about. many children lead normal lives with this type of congenital heart condition. No medications are indicated.
It is true, a muscular VSD of only 2.5 mm has an excellent chance of closing on it's own without any surgery or medications. It's a very common problem, and small defects do commonly close or get smaller on their own.