Never heard much about Vectra 3D but I have to say... I am not a Frontline fan anymore. It just does not seem to work around here and I see you are fairly close to me in MD
Jackson used to get ticks ALL the time, even got Lymes once. This was the summer of 2010, I believe, and he was on Frontline all summer, but I still was always finding ticks. I switched to K9 Advantix and the problem went away. In summer '11, I gave him one dose of Advantix in June and then did not give him anything until October, did not have a problem with ticks all summer, we maybe saw one. Anyways, so in October, gave him the Frontline dose, then in November, gave him again. Suddenly, I'm getting one tick a week off him. I REALLY am thinking they are attracted to the stuff or something! It's crazy. So he has not had a FL dosage since first week in November and all of Dec and Jan has been fine (and as you know, it's been unusually warm...)
So anyways, some are not comfortable with the permiterin (sp?) in Advantix but I feel more confident using it maybe once every 3 months and having it last, vs. using Frontline every 2 weeks (which seems to be the max it lasts until I begin seeing ticks).
I do use heartworm medicine all year and find it much more important.
I am definitely curious about the Vectra 3D as our vet carries it too.