5200 would be as low as I would go, from experience tuning my b16a2 which vtec engages at 5600 with it lower to 5200 it smooths out the enagement, which some say is good and some say bad b/c vtec isn't as noticable the dip before vtec is there so you feel it hit.
This is my dyno w/ 5200 vtec engagement and some air fuel tuning w/ a vafc.... The red is before tuning the air fuel but w/ the 5200 rpm enagement, and the blue is after air fuel tuning and 5200 rpm engagement. The car had I/H/E only.


Heres a b16 dyno the lower line is in stock form see the dip at vtec?

Lowering enagement will only smooth the transition out to a point then the lower you go the car doesn't run right at all.