It depends on the circumstances, vets know that many dogs don't like them, some see sgressive etc. the clinics have various styles of muzzles for the dog to wear and they ask the clients if the dogs aggressive or fearful. If the clients i.e. And the dogs uncontrolled the vet can try and sedate the dog and or muzzle it. Most vets won't endanger their staff with a vicious dog, they have the right to refuse a cienr.
They kind of expect that dogs are on their worst behavior at the clinic, they are anxious and afraid, ther are strangers there, strange dogs and in general the dogs at it's most stressed out time. It can show a different personality then it normally has. If
The vet usuay asked if the dogs agessive or has tried to bite, if the owner refuses to help control the dog then they ma sedate it or muzzle it. My vets been bitten severe times but nothing too serious.
The exception is if your at the vets office and say 'get him', making the dog attack and the dog attacks, then it's considered asset and yes the vet can minimaly impound the dog, then have the owner arrested for assul.
Also if the dog proves it's rabid or deemed a dangerous dog the vet can destroy the dog, though I thinks it's through a court order.