Dear Lia,
It sounds like you have a lot of good information to start with, in that the VSD is perimembranous and is 3.5 mm in diameter. If she is doing well now, i.e. eating well without getting out of breath or getting sweaty with feeds, it is likely that the defect is small and is restrictive to extra blood flow recirculating through the hole. This means that she should be able to grow relatively well. Obviously, without seeing her, I cannot say for sure how she will do. However, there is a reasonable chance that this defect will be closed, or at least will be somewhat smaller. If it is a small defect, it is not necessary to close it UNLESS the aortic valve above it starts to function abnormally, such as having one of the leaflets pulled down into the defect and having the valve leak. Otherwise, there are plenty of grown people with normal lifespans who have small VSDs that are not limited. Your cardiologist should be able to discuss how she is doing overall, though, and ensure that she is not getting in trouble.