Astronomos drifted from $4.60 to $7.50 in a 1500 metre handicap at Rosehill on August 15 before finishing ninth of 10, more than 3-1/2 lengths from the winner Dylan's Rojo.
Although the performance fell short of some expectations, Vandyke and rider Jason Collett told stewards there were mitigating factors.
The horse was coming back 500 metres in trip five weeks between runs and was forced to make a prolonged run from back in the field.
Vandyke said Astronomos simply "got too far out of his ground” and the sectional times from the race paint a positive picture for a horse stepping back up to 2000 metres on Saturday.
Astronomos recorded the fastest final 1200 metres of the 1500 metre journey with a time of 69.36 seconds but it was the final 1000 metres that caught the attention of Vandyke.
"I was happy with the run, of the 54 runners that contested races beyond 1200 metres that day, he ran the second fastest final 1000 metres,” Vandyke said.
The final 600 metres for the race was clocked at 34.27 seconds with the Dylan's Rojo leading from start to finish, so mathematically it would have been almost impossible for Astronomos to bridge the gap.
Astronomos has drawn barrier nine of 10 in the TAB Rewards Handicap on Saturday with Collett retaining the ride.
Vandyke has signaled the Group I The Metropolitan (2400m) at Randwick on October 3 as the spring target for Astronomos.