While Viljoen recorded figures of 6 for 93 in 28 overs, Deacon took 4 for 46 to make sure the Lions made the most of winning the toss and opting to field first.
They did receive a blow late in the day, however, when Stephen Cook shouldered arms and was bowled by Andrew Birch to leave the Lions none for one at the start of their reply.
With Port Elizabeth enjoying a fair amount of rain recently, Cook’s decision to field first appeared to be a straightforward one - particularly when the Warriors fell to 18 for three inside eight overs.
Michael Price was the only Warriors batsman to stand firm at the top of the order, and he eventually found some support in Colin Ingram and then in Bailey.
Price and Bailey put on 86 for the fifth wicket before the former was caught behind off the bowling of Viljoen three runs short of his century.
Jon-Jon Smuts contributed a run-a-ball 35 as the Warriors looked to add quick runs before the second new ball - a tactic which proved wise when Viljoen and Deacon ripped out the last five wickets for 32 runs in nine overs.
Bailey was left stranded on 102, and there was just enough time for one more over in the day.
Birch made sure that it counted when he removed Cook with the fifth delivery to give the Warriors a window of light at stumps.