stall speed
A 'certification' stall, i.e. an IAS decrease at the rate of 1kt/sec as opposed to a 'training' stall where level flight is maintained until the manifestation of the stall, will produce two different cockpit ASI readings for the two stalls. It is some time since I have flown a C172 but I imagine that the ASI does not indicate much below 30kts? If this is the case, an accelerated entry into the stall, not an 'accelerated stall' but a decrease in IAS of >1kt/sec may result in an unreadable IAS indication before the 'stall' IAS is reached. Particularly if the the VGs have lowered the stall CAS/IAS by, as you say, 'a few knots'. Tailplane control surface effectiveness is dependant on Reynolds number amongst other things but also energy recovery from the airflow off the wings and the propellor. Lower speed, less energy recovery, less effective the tail surfaces. Hence the 'mushy' controls.