I don't think any dogs are "vicious". I think a lot of dogs, particularly small dogs, live their days confused and afraid and respond in the only way that has worked so far for them- they snap at what is scaring them. The sad thing about small dogs is that their owners perpetuate their fears by not treating them like dogs- if they had socialized and trained the dog, the dog would not be so fearful and would understand how humans communicate with dogs (training). "spoiling" dogs is very scary to them- rules are very comforting to dogs.
As others have said, certain breeds may be more inclined to being fearful or assertive, more inclined towards responding to certain stimuli with a bite or a bolt, but true "viciousness" is not a breed trait any more than viciousness is a trait in particular ethnicities. What we think of as viciousness is a wiring problem in a very limited number of individuals.