What each option actually delivers
A VAN moves documents between you and your trading partners. That is valuable and it is not integration: something still has to translate the document and post it into NetSuite correctly. Teams that buy a VAN and assume ERP posting is included discover the gap during their first partner go-live.
A SuiteApp keeps translation and posting inside NetSuite, which is convenient for admins and ties your EDI capability to your ERP configuration. A managed service takes the whole path — connection, translation, posting, acknowledgement, and monitoring — and puts one team behind it. In-house maps on an integration platform give you complete control and the full maintenance load.