Vicious means (1) evil, immoral, or depraved; (2) spiteful, malicious, (3) aggressive, and (4) severe or intense.
Viscous means having relatively high resistance to flow (think mud or industrial sludge).
Gallons of the viscous amber fluid have been given to politicians and journalists, as the apiarists of Australia lobby for action against the imminent incursion of Asian bees. [Sydney Morning Herald]
The thick and heavy bitumen, sometimes called tar or tar sands, is too viscous for traditional pipeline transportation. [The Cherokeean Herald]
Once there, it will reduce the viscosity of the thick crude oil residing in the well and allow for easy extraction. [Inhabitat]
Minks are vicious carnivores. [NPR]
An ordinance providing for the regulation of vicious and potentially dangerous dogs was introduced on first reading by the township committee this week. [NJ.com]
I defend their right to be vicious and I don’t take it personally anymore, because I see literary viciousness as a dark art that sometimes needs writers as its canvas. [The Guardian]