The teenage substitute fired in an excellent right-footed 20-yard half volley in the fourth of six minutes of stoppage time to net his first senior goal, soon after Giles Barnes had done likewise to seemingly put Derby on course for a shock away victory.
But the Rams ultimately paid the price for their delaying tactics, particularly in the second half, which largely succeeded in frustrating the Hornets in their attempts to gain the victory that would have moved them within five points of second place in the Championship.
Watford started in a manner which suggested they could win a ninth league game in 10 and Ashley Young should have given them an early lead.
The winger was denied by Lee Camp after being put through on goal but then glanced an even better chance wide of the target after being picked out by Marlon King's follow up cross.
Instead it was Derby who drew first blood when the Hornets offside trap failed and on-loan striker Kevin Lisbie broke clear.
Although Jordan Stewart got back to win a challenge, the defender then appeared to slip and Lisbie managed to squeeze a shot beyond Ben Foster.
Watford then struggled against the visitor's five man midfield but equalised in real style with the Championship's leading scorer King netting his 18th of the season.
The former Nottingham Forest frontman collected a long Foster free-kick on the edge of the Derby area and held off Darren Moore before creating space to slam a low 20-yard drive past Camp.
Hameur Bouazza should have put the Hornets in front before half-time but blazed badly over from just six yards out.
Watford bossed most of the play after the break, but increasingly became frustrated by Derby's time wasting tactics which manager Terry Westley was later to describe as "bonkers".
Watford continued to push and it looked like Derby would snatch the points when, four minutes from time, a long throw was flicked on by Adam Bolder to Barnes who confidently beat Foster from 18 yards.
However, Watford have shown on several occasions this season that they do not know when they are beaten and this characteristic came to the fore again in injury time when Bangura equalised.