Vegeta lets Cell try and absorb Android 18 to reach his Perfect form. Android 16 tries to stop him but is defeated. Trunks tries to intervene but Vegeta stops him. Cell attacks #18 and absorbs her reaching his perfect form and defeats Krillin to test his power.
If Cell defeats Trunks then he proceeds to battle #18 with Vegeta not appearing anywhere in the battle. If Cell defeats #18 before she switches then Vegeta will watch Cell transform.