Vorster got support from Gordon Reisima (28) and Gus Borren (46), and while Wanganui bowler John Beale (3-39) eventually got through to the century-maker's stumps, the damage was done at 323/9.
"We bowled pretty poorly," said coach Warren Marr.
"Some naive bowling and not sticking to the plan."
In addition, there were six catches put down, three of them off Vorster.
"It's disappointing to drop catches, and guys that usually taken them. There were nicks," said Marr.
Connor O'Leary was again the workhorse bowler with 3-82 from his 22.3 overs while, as expected, the returning Ben Smith sent down some off-spin and finished with 2-30.
In the field, Wanganui opening batsman Chris Stewart suffered a dislocated shoulder going for a big drive, requiring an ambulance to be called.
Marr said the veteran had a good prognosis for recovery.
With nine overs left in the day, the coach then offered Smith, the leading batting hope, the chance to stay in the shed while Wanganui used a night watchman, but Smith was happy to pad up.
"I can't say enough good stuff about Ben," said Marr.
Unfortunately, Fraser Kinnerley was still required for night watchman duty when Hadleigh O'Leary was dismissed with five overs remaining, joining Will Hocquard and the injured Stewart.
Smith and Kinnerley saw their side through to the end of play, but the heavy rain and covers which had holes and taped-up sections failing to keep the water out meant Wanganui will never know if their Stags batsman could have carried them to 344 the following day.
Meanwhile, Property Brokers United First XI booked their place in the Rep Weekend Pro40 final with one round still remaining after victory at Tasman Tanning (Victoria) Park on Saturday.
United picked up a 25-run win over Tech Old boys, despite looking vulnerable when they were bowled out for 134, as Tech's Deek Reuters took five wickets.
However, spinner Robbie Power picked up a four-for, as Tech were bowled out with nine overs remaining for 109.
