NASCAR use what they term a "Competition Yellow". After 'x' amount of laps if there has been no warranted caution (wreck, debris on the track), they throw the yellows out, bunch the field up, allow cars to pit and allow cars to get back onto the lead lap with a free pass for those cars a lap down. They claim it is to prevent dangerous tyre wear but the consensus among the fan base is that it is done to bunch everybody up and prevent a few of the lead drivers breaking away. I've seen the them throw it out at Watkins Glen, a normal road course, at which Juan-Pablo Montoya & Marcus Ambrose (of Aussie V8 Supercar fame) were crushing the field.
On Sunday those safety cars worked in the same above way; they bunched the field up. Vettel would have probably had his way with the lower-half of the midfield, but without safety cars the top guys would have stretched away from the field making his blast onto the podium less likely. Remember he alse had to change his wing and suffered little to no time penalty. He went to the back, but due to the safety car he was right there and with much slower cars in front.
If there was no safety cars my guess is he'd probably have been held up in the mid to lower points melee involving the likes of Williams, Sauber & Force India, cars that are no slouches and drivers that would have made life a lot harder for him, especially with the brain fades that characterised the racing among the drivers of those respective teams on the day.
Obviously those yellows on Sunday were warranted, but to say they didn't make Vettel's surge through the field easier is incorrect.