Watched by a crowd of 626, some no doubt attracted by the offer of free admission, City got off to the perfect start when they took the lead in the 16th minute.
The lively Matt Jay did well to win the ball in his own half and he launched a long ball to Vassell on the right and the in-form striker cut inside before letting fly with an unstoppable left footed drive into the far corner.
But the home side then switched off seven minutes later as they allowed Hemel captain Jamie Slabber a clear path to goal and he gave Martin Rice no chance with an assured finish.
City though continued to hold the ascendancy, producing some great attacking football, with the pace of Vassell causing the visitors continual problems.
And when, after 28 minutes, the striker latched onto another long ball, he was brought down just inside the area by a desperate tackle by Hemel defender Oliver Thorne who was booked.
Brett took the penalty and although Hemel keeper Jamie Butler got his hands to spot kick as he dived to his left, the ball found the back of the net via the post.
In the second half though City started to sit deeper, allowing the Hertfordshire side to push forward and several times they threatened a goal with Rice making a couple of vital saves
So it was a great relief to the home fans, who had seen the team throw away a two goal lead three times in their previous home match against Maidenhead United, when great work by Vassell saw him win the ball just on the half way line