I have not landed in Vallentuna, but it's a good field. A friend's father used to keep (and maybe still does) his old DC-3 there! Vallentuna is reasonably close to Stockholm - you can be in town in about 1hr.
You can chose not to use Grafair at ESSB and because I just checked, the prices are as follows:
Landing 250 Skr.
Parking 281 Skr/24hr period.
But they do charge you a to- and from apron fee of 450 Skr (you're not allowed to walk), which is kinda criminal. But all in all, perhaps worth it for a shorter visit. Would put it around 1000 Skr if you stay one day, which is compared to many other airports in Europe not that bad. Parking will for sure be cheaper at Skå Edeby if you plan on extended stay...