!!!
If your ECU Is not getting a temp reading, it will not engage VTEC!!!
One of the checks it does is to make sure coolant temp is @ a certain threshold before it allows VTEC to engage
(the other checks being vehicle speed (>15mph), oil pressure, RPM, TPS)
Might be a pain in the ass at this point, but a good spot to tap for water temp is the bottom of the radiator. In the B-series, coolant flows top->bottom. Also a good spot to tap for a water cooled turbo.