Personally, I think they are different. However, only relative to the effort involved. Both VM and WOM are spread by people that are happy about something or really unhappy about something.
Word of Mouth takes a gallant effort to spread the word.
"Hi Jane. My friend just opened a purple horse store. Would you like to go to it?"
Viral Marketing takes less effort (if any) because the subject matter is fascinating and irresistible to spread to friends and family. Because of how quickly it spreads, the control factor is in the toilet. As all of you know, many companies do their best to "find" that next viral campaign only to have it backfire.
Creating something that is irresistible to talk about can be difficult. I have had some success when I:
- Define My Target Audience
- Define My Target Market
- Make sure these are both the same or at least enough to make the campaign viable
- Find two polar points within the Target
- Pick a side and
- Then... challenge the other sides beliefs and/or behaviors