We looked into VHT but it just didn't make sense when we could get perfectly fine traction out of Mountain Dew. I'll say it again: to anybody serious about running a parking lot series, this is the magic combination:
1) seal the lot every 2 years ($300-400 for a typically sized racing surface)
2) use a Billy Goat Blower ($600 new) to blow off the track
3) use garden sprayers to put down about 10-12 2L bottles of Mountain Dew in the run line ($20 per week)
With that combination, you will never be hurting for traction. Obviously not everybody can be that serious, but after a lot of experimentation we could not ask for a better racing surface. And best of all, most lot owners will be quite happy when you offer to come out and re-seal their lot for free; others might even pay for the sealant if you are doing the work for free.
I should note that the air temperature went between 75* and 95* during our various events last year with this setup, and even at the lower end of that, there was still plenty of bite.
The sealant is not essential but it really helps tire wear, especially on nitro cars and foam tires.