EVERY fixture needs a vent. Either a dry vent or a wet vent.
Depending on your local codes there are quite a few rules to wet venting. One of them is that YOU CANNOT wet vent passed a water closet. That is why toilets are always tied in LAST on a wet vent system. So if you were to add more fixture you would need to tie those fixtures in upstream of the toilet connection.
Better you you create an additional wet vented system through the new vanities and tie that new system in downstream of the existing toilet and wet vented system.