Well, if Cell is still willing to fight a full power Goku, Vegeta should be smart enough to realize Cell still has big reserves of power, no? Unless they all think that weakened Cell (which was far from the level he begun fighting 100% Goku with) was powerful enough to take out Goku post the Senzu Bean, which seems kind of ridiculous imo. So if Vegeta thinks Cell is still hiding his ture power, then it's possible he was taking that into account when saying no one there has a shot at beating Cell, considered such statement was made after Cell motivated Goku to be back at full power.