The Seattle Sounders are the only American club to be in the A-League for all of the last 10 seasons. Check out the Sounder Central museum pages celebrating this accomplishment. http://soundercentral.com/museum/A-LeagueSounders.htm The other 10-season clubs? Montreal, Vancouver and Toronto! (I think they didn't miss any years...)
Montreal sat out one season (1999?). Toronto Lynx's first season in the league was 1997 so I don't know what you are thinking of there.
In 1994 the Sounders played the Toronto Rockets in the APSL. I was looking at that and assumed the Rockets eventually became the Lynx. I see now that Toronto had a gap between teams, and Montreal did indeed sit one out in 1999! http://www.sover.net/~spectrum/year/1999.html#aleague So, it's just Seattle, Vancouver!