Vassell bagged a first-half hat-trick in the 5-1 win over Bradford Park Avenue on Wednesday, adding to the goal he scored against FC Madgeburg last weekend.
The former Blackpool man suffered a disappointing second half of last season, with injury ruining a fine start to his Millers career which had seen him win a maiden international call-up for Northern Ireland.
The 26-year-old, who was used predominantly as a wide man last term, prefers to play as a striker and has seen Freddie Ladapo and Carlton Morris come in this summer, adding to the fit-again Jamie Proctor and first-choice Michael Smith in the ranks.
But he is using his opportunity to show Warne that he should be in contention for a striking role and the boss has been impressed.
“He has been excellent, he has been one of the standout players of pre-season, for sure,” Warne said. “He has been excellent in training, when we had the games in Germany he was my first pick which is hugely impressive.
“He was a real livewire at Bradford, he came into pockets and most of our good play was through him really.”
The Millers have been put through their paces so far, having had nearly 80km of running put into their legs in the week-long training camp in Germany, while there has been no let up since their return to England.
Warne is happy with where his side are at ahead of remaining friendlies against Chesterfield, West Brom and Leicester.
“The lads physically are doing really well, but mentally we have got to sharpen up and technically we have got to sharpen up, we are not bang on it,” he said.
“I think we are seven and a half out of 10 at the minute, hopefully we can get up to 10 by the time of the Wimbledon game.
“We have got Chesterfield on Saturday and than have got two massive out-of-possession tests against West Brom and Leicester. That will all add to the sharpness.
“All the games bring something. I asked the lads to put their names on the teamsheet and some of them did that.
“Pre-season isn't about winning games, it is just about getting them understanding how we play and trying to get them into a fit state so they can play and at the moment I am pleased with how that is going.”