i have never had a problem dropping out of vtec with my h23a1 tranny.
its never happened
however having driven both tranny's in one weekend, id say that the h23a1 doesnt line up with VTEC as much as the h22a/1 tranny. the h22a tranny just comes in almost exactly AT vtec.

and i DID occasionally drop out of VTEC in the h22 tranny. because u are shifting at redline to hit enagement in the next gear. and if u shift below redline by too much then u end up bogging at like 4500 in the next gear.

i prefer the h23a1 tranny. in fact i swapped the h22a tranny in the weekend i did my ACT clutch upgrade and then out the next weekend, in favor of the h23a1 tranny.

my preference is also based largely on the fact that my average cruising speed on the parkway/expressway is about 80-85, and that is around 4000 rpms in 5th gear.

~boom