Because there was no vigorous fermentation does not mean it's not finished. Like i said my honey weizen never had the vigorous fermentation, but it did reach close to the expected final gravity...only .002 above what I expected, so I call that good. You can go by what FG it said in the kit. The vanilla won't change the FG enough to worry about if at all.
the OG isn't nearly as important and the FG. You only need the OG when you are figuring ABV. Final gravity for most beers should be around 1.010-1.012 or lower for most beers..higher for some if it will be a sweet beer. I brewed the Chocolate Covered BEAVR Nutz and the FG I reached was 1.020, which is where that one was supposed to finish. That one didn't have a vigorous fermentation either, but it finished where it needed. You don't need to see a lot of airlock activity for fermentation to take place.
Do a search on stuck fermentation...in most cases, people say that pitching more yeast after fermentation has started will not work because the alcohol in there will kill most the yeast. Will it hurt? probably not, but if you don't know the SG that you are at now, then you can't tell for sure if fermentation is done or where it is at.