RNW, my little dd had open heart surgery at Evelina Children's Hospital 9 years ago when she was 4 months' old, also for a large VSD.
I can hth say your ds will be in the best of hands.
I cannot remember what I brought in terms of things.
I brought someone to soothe my dd because she will not be allowed to eat for many hours ahead of surgery. I knew she would cry but I would go to pieces. She was bf-ed and smelling the milk on me would set her off so MIL rocked her off to sleep instead of me. Funnily, dd did not cry as much as I thought she would. Maybe she knew.
The worst part is carrying your ds down the corridor and then giving him over to the anesthetists and for the doors to close. That is the absolute worst bit, so be prepared for it and make sure your dh gives you extra support then. After that, it is out of your hands.
The next 4-5 hours during the surgery, just leave the hospital and go to a cafe nearby or anywhere that will distract. I believe the hospital will page you once the surgery is over. So bring anything/go anyway that will get you and dh through that.
When you first see ds, he will be full of tubes and wires and slightly bloated. But the tubes and wires will come out very quickly one after another within 24-48 hours, I believe. As for bf-ing, I think it was only the initial day or so I had to express. Once dd was awake enough, I could just put her to the breast direclty. It will be a huge comfort for you and ds to bf. I was never so glad of it.
I was amazed that I was allowed to lift dd so soon after her surgery. I thought it would hurt her but that was not an issue. Be aware that the pain relief is very good and they keep the little ones very comfortable.
The great thing is that VSDs, even big ones like your ds and my dd's, are eminently fixable. Dd has been completely repaired since the surgery and apart from the scar on her chest lives like any other child. It is a non-issue.
Evelina is fantastic. Hang in there and all the best! It is worse for his parents than it is for your ds.