Retainers or night guard with veneers? Could a nightguard serve as a retainer?

If the impression can loosen your veneers, then your veneers aren't very good.  If removing the nightguard can loosen your veneers, they aren't very good.  A nice, hard acrylic nightguard can also act as a retainer, so I would get one ASAP.If your dentist uses an iTero (or ANY digital impression system), there won't be any way to loosen your veneers.

In my practice I never give a veneer patient a night guard or retainer after veneers. The reason for this is night time bruxism is all bout a lack of oxygen. So, if you are grinding your teeth then adding a piece of plastic to your mouth creates yet another hurdle for oxygen to pass. 
Now to answer your question. If the impression can loosen your veneers, then there’s something wrong with them