Held items that affect you which are disabled in dynamax are:
Choice Band, Choice Scarf, Choice Specs, Gorilla Tactics, and Sheer Force temporarily stop working (including the stat boosts) if the holder is Dynamaxed. However, Assault Vest still works accordingly and prevents the holder from using Max Guard.
So, the Choice items, i.e. Choice Band, Choice Specs and Choice Scarf are disabled when you dynamax, and Assault Vest isn't one of those and is enabled, and prevents holder from using Max Guard. Also, you aren't flinched, but can be statused, etc. but those are move effects not item effects, so they aren't included.
Hope this helps!